Set target ruby version for rubocop to 2.2 instead 2.3.

This change will help to catch ruby 2.3+ syntax in code base. Right now, minimal ruby version set to 2.2.2 in gemspecs.

Rubocop output before:

$ rubocop
Inspecting 2133 files
.............................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................C............................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................CC.........................................................................................................................................................................................................................................................

Offenses:

activerecord/test/cases/schema_dumper_test.rb:454:32: C: Prefer double-quoted strings unless you need single quotes to avoid extra backslashes for escaping.
    output = dump_table_schema('infinity_defaults')
                               ^^^^^^^^^^^^^^^^^^^
guides/bug_report_templates/active_record_migrations_gem.rb:65:4: C: Final newline missing.
end

guides/bug_report_templates/active_record_migrations_master.rb:64:4: C: Final newline missing.
end

2133 files inspected, 3 offenses detected
$

After:
$ rubocop
Inspecting 2133 files
.............................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................C............................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................CC.........................................................................................................................................................................................................................................................

Offenses:

activerecord/test/cases/schema_dumper_test.rb:454:32: C: Prefer double-quoted strings unless you need single quotes to avoid extra backslashes for escaping.
    output = dump_table_schema('infinity_defaults')
                               ^^^^^^^^^^^^^^^^^^^
guides/bug_report_templates/active_record_migrations_gem.rb:65:4: C: Final newline missing.
end

guides/bug_report_templates/active_record_migrations_master.rb:64:4: C: Final newline missing.
end

2133 files inspected, 3 offenses detected
$

.rubocop.yml

 AllCops:
-  TargetRubyVersion: 2.3
+  TargetRubyVersion: 2.2
   # RuboCop has a bunch of cops enabled by default. This setting tells RuboCop
   # to ignore them, so only the ones explicitly set in this file are enabled.
   DisabledByDefault: true