[ci skip] Minor improvements on routing.md.
Merge pull request #15029 from JuanitoFatas/docpatch/routing

[ci skip] Minor improvements on routing.md.
Change 'a' to 'an' [ci skip]
Merge pull request #15030 from mechanicles/a-to-an

Change 'a' to 'an' [ci skip]
add tests for the polymorphic_path method
add a test for classes with options
don't manipulate the testclass, just add helpers to the instance
add tests for path based url_for calls
Use ruby 2.1.2 on travis
Merge pull request #15041 from arthurnn/update_ruby

Use ruby 2.1.2 on travis