[build-troubleshooting] Dropping travis support for jRuby and Rubinius.

I only added these because they seemed to be there by default.

For jRuby, I think something's up with how the 'env.rb' in my
cucumber/support directory is modifying the load path/bin path with
jRuby.

As for rbx.. I don't know.

Here's what's motivating this change:

jRuby: https://travis-ci.org/jedcn/reveal-ck/jobs/4664834
rbx: https://travis-ci.org/jedcn/reveal-ck/jobs/4664835

Not interested in troubleshooting this right now (or ever?) for this
dinky little tool.