Enable Rubinius 2.1 build on Travis CI

http://rubini.us/2013/12/03/testing-with-rbx-on-travis/
https://github.com/yujinakayama/guard-rubocop/commit/0438ebc0099805b8bc923a09b73df1741c26ff96
https://github.com/bbatsov/rubocop/commit/c2906d188d98885a96ea3dc498f643664d68d74a