Travis: jruby-9.1.13.0, jruby-1.7.27

[olleolleolle]

This PR updates the CI matrix to use latest JRuby.

http://jruby.org/2017/09/06/jruby-9-1-13-0.html

[kares]

Thanks for your effort Olle, since the CI build is broken already I am not sure it makes sense to upgrade.
The gem used to be somehow sensitive to JRuby changes, in the hopes of someone eventually picking the work up (to upgrade to latest rhino version) would rather keep the merge open until it first passes with the old versions.