Added rake to Gemfile under test.

This eg permits good command line usage like:

  rbx -X19 -G -S rake test

which loads the Gemfile and runs rake test, instead of needing

  bundle exec rake test
Test on Travis CI.
Ignore Rubinius compilation cache directory.
Merge pull request #1 from brixen/master

Test on Travis and a couple Rubinius-related tweaks.