Move Rubinius dependencies out of .gemspec
Rubinius users will have to include them manually (see http://www.benjaminfleischer.com/2013/12/05/testing-rubinius-on-travis-ci/ including References)

Also, add missing rubysl-rake dependency (which, I guess, was the cause of this test suite failure : https://travis-ci.org/michaelbaudino/addic7ed-ruby/jobs/16382973)

Related to c85fab204f16fd629ce7a0dc173844dc44ec31ff