Reverse 821525e and wrap run_generator call

Ruby 2.1.0 includes the json gem 1.8.1 by default so we need bundler 1.5.1
for `bundle install` to work. To fix this reverse the downgrade to 1.3.5
and wrap the `run_generator` call with a block that resets `THOR_DEBUG`.

.travis.yml

@@ -1,6 +1,6 @@
 script: 'ci/travis.rb'
 before_install:
-  - travis_retry gem install bundler -v 1.3.5
+  - travis_retry gem install bundler
   - "rvm current | grep 'jruby' && export AR_JDBC=true || echo"
 rvm:
   - 1.9.3

railties/lib/rails/generators/testing/behaviour.rb

@@ -61,11 +61,13 @@ def destination(path)
         # You can provide a configuration hash as second argument. This method returns the output
         # printed by the generator.
         def run_generator(args=self.default_arguments, config={})
+          without_thor_debug do
             capture(:stdout) do
               args += ['--skip-bundle'] unless args.include? '--dev'
               self.generator_class.start(args, config.reverse_merge(destination_root: destination_root))
             end
           end
+        end
 
         # Instantiate the generator.
         def generator(args=self.default_arguments, options={}, config={})
@@ -100,6 +102,14 @@ def migration_file_name(relative) # :nodoc:
             dirname, file_name = File.dirname(absolute), File.basename(absolute).sub(/\.rb$/, '')
             Dir.glob("#{dirname}/[0-9]*_*.rb").grep(/\d+_#{file_name}.rb$/).first
           end
+
+          # TODO: remove this once Bundler 1.5.2 is released
+          def without_thor_debug # :nodoc:
+            thor_debug, ENV['THOR_DEBUG'] = ENV['THOR_DEBUG'], nil
+            yield
+          ensure
+            ENV['THOR_DEBUG'] = thor_debug
+          end
       end
     end
   end