Add ruby-head to .travis.yml

[aycabta]

Description

It's important for tests because YARD uses Ripper for parsing Ruby code and Ripper is sometimes updated. For your action, RDoc is tested on ruby-head because RDoc is using Ripper now too.

Completed Tasks

• I have read the Contributing Guide.
• The pull request is complete (implemented / written).
• Git commits have been cleaned up (squash WIP / revert commits).
• I wrote tests and ran bundle exec rake locally (if code is attached to PR).

[coveralls]

Coverage increased (+0.02%) to 93.52% when pulling 4f59d14 on aycabta:use-ruby-head into 7748eda on lsegal:master.

[aycabta]

Oh, OK, I'll fix tests of YARD for Ruby trunk later. Recent several months, Ripper has many commits.

[lsegal]

Seems like there are regressions in ripper that Ruby core should be aware of here.

[aycabta]

I created this Pull Request just after I read #14126 on Ruby trunk (ruby-core:83864) that is closed by revision 60892. I made sure of that revision 60891 is failed and revision 60892 is correct on YARD. The cause of tests failure of this Pull Request is the revision of ruby-head on Travis CI was delayed. So, please rebuild the tests on Travis CI. My branch is correct after ruby-head revision is updated.

[aycabta]

Note: This is not for latest Ripper behavior. This is for determining if YARD follows latest Ripper's behavior because of Ripper sometimes changes the behavior. I'm discussing some changes to Ripper with Ruby core team, so I guess this is important.

[lsegal]

I agree that this is important. I do hope that backward compatibility is kept in mind though. Ripper can add things, but plenty of things rely on its current behavior, and any type of changes should be backwards compatible, especially since the library is part of core.

Thank you for improving Ripper!

[aycabta]

I understand your concerns. I think so that backward compatibility is so important because I'm the main maintainer of RDoc 6 that uses Ripper, so I treasure backward compatibility of core libraries too. Thank you for your mind, I promise that I pay attention to it.