Use inline Gemfile dependency when reporting bugs

With Bundler `1.10.3`, it is possible to list the gems inline without the
need to create a `Gemfile` if none is found in the current directory.

guides/bug_report_templates/action_controller_master.rb

-unless File.exist?('Gemfile')
-  File.write('Gemfile', <<-GEMFILE)
-    source 'https://rubygems.org'
-    gem 'rails', github: 'rails/rails'
-    gem 'arel', github: 'rails/arel'
-  GEMFILE
-
-  system 'bundle'
-end
+require 'bundler/inline'
 
-require 'bundler'
-Bundler.setup(:default)
+gemfile(true) do
+  source 'https://rubygems.org'
+  gem 'rails', github: 'rails/rails'
+  gem 'arel', github: 'rails/arel'
+end
 
 require 'rails'
 require 'action_controller/railtie'

guides/bug_report_templates/active_record_master.rb

-unless File.exist?('Gemfile')
-  File.write('Gemfile', <<-GEMFILE)
-    source 'https://rubygems.org'
-    gem 'rails', github: 'rails/rails'
-    gem 'arel', github: 'rails/arel'
-    gem 'sqlite3'
-  GEMFILE
-
-  system 'bundle'
-end
+require 'bundler/inline'
 
-require 'bundler'
-Bundler.setup(:default)
+gemfile(true) do
+  source 'https://rubygems.org'
+  gem 'rails', github: 'rails/rails'
+  gem 'arel', github: 'rails/arel'
+  gem 'sqlite3'
+end
 
 require 'active_record'
 require 'minitest/autorun'

guides/bug_report_templates/generic_master.rb

-unless File.exist?('Gemfile')
-  File.write('Gemfile', <<-GEMFILE)
-    source 'https://rubygems.org'
-    gem 'rails', github: 'rails/rails'
-    gem 'arel', github: 'rails/arel'
-  GEMFILE
+require 'bundler/inline'
 
-  system 'bundle'
+gemfile(true) do
+  source 'https://rubygems.org'
+  gem 'rails', github: 'rails/rails'
+  gem 'arel', github: 'rails/arel'
 end
 
-require 'bundler'
-Bundler.setup(:default)
-
 require 'active_support'
 require 'active_support/core_ext/object/blank'
 require 'minitest/autorun'