Also drops the unused cucumber-pro token

By letting Travis CI handle Bundler, caching will be automated.

- Use latest point release for CRubies starting from 2.2
- Drop 20 and 21 modes for JRuby. These do not seem to do the right
  thing and instead just test on JRuby 9.2
- Add 1.8 more for JRuby 1.7 via JRUBY_OPTS variable

This was based on rspec's Travis CI build matrix.

There are two reasons for this:
- Coverage is inaccurate unless running in debug mode
- Simplecov breaks on JRuby 1.7 in 1.8 mode

This will also load the old childprocess version on CRuby 1.8, but the
main point here is to use it with JRuby with the --18 flag, since it's
the JRuby-specific code in childprocess that has an incompatibility.

Something seems broken on 9.2.7.0 which is not broken on 9.2.6.0.