Update ruby, rails version for travis builds

[lostapathy]

This updates the travis config for the latest ruby and rails versions.

• --skip-listen was added because travis was having intermittent failures with the Guard gem
• --skip-system-test was added because it creates a dependency on capybara in the generated app which does not resolve in a buildable way with ruby versions less than 2.3.

I don't believe either of these changes affect the utility of the test suite.

[rails-bot]

Thanks for the pull request, and welcome! The Rails team is excited to review your changes, and you should hear from @jonleighton (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[lostapathy]

The current travis failure seems to be an issue with their build system. While testing, I saw this build fail this way about 25% of the time for no apparent reason. It will most likely run if someone with the correct access tells it to retry.

https://travis-ci.org/lostapathy/spring/builds/434817233 - I setup travis on my fork and the same code built fine here.