Add Ruby 2.0/2.1 to CI and fix deprecation warnings

- Adds Ruby 2.0 and 2.1 to Travis CI
- Drops Ruby 1.8.7 and REE from Travis CI since the tests were [false positive](https://travis-ci.org/mdp/gibberish/jobs/11445541) due to [this 1.9 hash syntax](https://github.com/mdp/gibberish/blob/master/spec/aes_spec.rb#L94)
- Fixes deprecation warnings for Digest::Digest in Ruby 2.1
- Fixes deprecation warning for MiniTest::Unit.autorun