Fix rbx generated Gemfile regex

https://travis-ci.org/rails/rails/jobs/39190983#L460

 1) Failure:
AppGeneratorTest#test_psych_gem
[test/generators/app_generator_test.rb:495]:
Expected /gem 'psych',\s+'~> 2.0', \s+platforms: :rbx/ to match <snip>
http://git.io/uuLVag)\ngem 'psych', '~> 2.0', platforms: :rbx\n\n