Travis: Create a separate RuboCop "Build Stage"

[olleolleolle]

This PR makes RuboCop run in a Travis Build Stage before the testing begins.

• This allows the test script to be bundle exec rspec && bundle exec cucumber.
• JRuby 1.7.27 gets to run only bundle exec rspec (it doesn't support the Cucumber suite)
• Drop RuboCop Rake task
• Drop Cucumber wrapper script
• Add latest available Ruby versions to CI matrix

See Travis' docs for examples of Build Stage matrix expansion usage.

This change enables us to upgrade to the very latest RuboCop, which is supported on Ruby 2+. (Beyond the scope of this PR.)

[olleolleolle]

There's probably a couple more splits that could be done; Build Stages have not yielded all its useful secrets yet!

[olleolleolle]

Hrm, I need to execute a string as a shell command. I'll get to the bottom of this!

[olleolleolle]

With the help of a trusted friend, I smote the dragon of complexity, cutting the size of the configuration and also of the CI matrix!

[egonSchiele]

Thanks for filing all these PRs! Awesome to have these contributions! 👍