Make tests pass under MRI 1.9.3, Rubinius and jruby
Fix Travis CI errors for Rubinius

Rubinius no longer includes the standard library as part of core, so standard library gems need to be loaded explicitly.
Fix Gemfile path
Tell Travis to install the coverage gem for rubinius
Install json gem for rubinius
Merge pull request #22 from allolex/multiple_rubies

Make tests pass under MRI 1.9.3, Rubinius and jruby