Improve travis build.

- No need to install bundler twice.
- No need to bundle install twice.
- Skip one-by-one specs on JRuby.
- Run cucumber using our standalone bundle
  (rather than bundle exec).
- Bundler issue #2383 has been resolved,
  no need to work around it anymore.
Revert how we run cucumber.

It's causing a SystemStackError on 1.9.2.
I'm not sure why and don't have time to debug it now.
Merge pull request #412 from rspec/improve_travis_build

Improve travis build.