Added rake to the bundle (Gemfile), as advised by this error message on Travis:
/home/travis/.rvm/gems/ruby-2.0.0-p594@global/gems/bundler-1.7.4/lib/bundler/rubygems_integration.rb:256:in `block in replace_gem': rake is not part of the bundle. Add it to Gemfile. (Gem::LoadError)