Add travis support. Adjust debugger gem requirements.

The Gemfile.lock forces (at the time of writing this) gem debugger-ruby_core_source version 1.3.2, which only supports ruby 2.1.0-p0. This version is forced at .travis.yml for ruby 2.1.0 and debugger's gem requirement is deleted for the rest of ruby versions (since debugger is never used in automated testing and makes tests slower).
Debugger gem is kept for 2.1.0-p0 so it can be used when manual testing at developing.