travis: drop old {,j}ruby versions and add JRuby 9.1.5.0
travis: drop old {,j}ruby versions and add JRuby 9.1.5.0

JRuby 9.1.1.0 is about the first version that could possibly work
with the dependencies we currently have because it added a proper
Process.getrlimit method.

3scale/3scale_ws_api_for_ruby