check .travis.yml ruby verison matches pages

[mlinksva]

#472 (comment)

very rough, naive, probably bad implementation

[parkr]

This is terrific! ✨ I would recommend also noting the process for bumping the CI Ruby version—should it be in a separate PR or in the same PR but separate commit?

[parkr]

I usually use the affirmative here, e.g. "Matches" rather than "doesn't match." I'm curious if you prefer the negative!

[mlinksva]

@parkr would you have process documentation be in test output, like (added text bold):

1. ci ruby version in .travis.yml and pages ruby version match
   Failure/Error: expect(ci_ruby_version).to eql(pages_ruby_version), msg
   (2.0.0) != (2.3.3) please add a commit bumping in .travis.yml

Or elsewhere?

[parkr]

In the test output is probably the easiest place!