Rewrite chunks of the Ruby documentation.

Keeps it more up-to-date with information relevant to most users
of Travis CI, incl. Ruby versions, some Bundler hacks and other goodies.

Removes a few things that were historically included and updated them
with writing that makes more sense with our current setup.
Add section on Bundler caching.
Add explanation how the caching works.
Slightly more detail on Rubinius.
Misc updates as per @henrikhodne's suggestions.
Fix slightly awkward speling.
Merge pull request #436 from travis-ci/mm-update-ruby-doco

Rewrite chunks of the Ruby documentation.