Run Active Support tests when preserving timezones

Add to the matrix of Travis tests an entry that runs the Active Support
tests when `ActiveSupport.to_time_preserves_timezone = true`.

.travis.yml

@@ -37,6 +37,7 @@ env:
     - "GEM=ac"
     - "GEM=ac FAYE=1"
     - "GEM=am,amo,as,av,aj"
+    - "GEM=as PRESERVE_TIMEZONES=1"
     - "GEM=ar:mysql2"
     - "GEM=ar:sqlite3"
     - "GEM=ar:postgresql"

activesupport/test/abstract_unit.rb

@@ -18,6 +18,9 @@
 # Show backtraces for deprecated behavior for quicker cleanup.
 ActiveSupport::Deprecation.debug = true
 
+# Default to old to_time behavior but allow running tests with new behavior
+ActiveSupport.to_time_preserves_timezone = ENV['PRESERVE_TIMEZONES'] == '1'
+
 # Disable available locale checks to avoid warnings running the test suite.
 I18n.enforce_available_locales = false