Include JRuby in Travis builds

Because I want to integrate this gem into a Maven plugin I want to be
sure that it will run on JRuby using various JDK implementations.

see GeoDienstenCentrum/sass-maven-plugin#3

Change-Id: I6394c0ea469370b953b3d5ad242d63afd4365f92
Reviewed-on: http://gerrit.causes.com/45471
Tested-by: jenkins <jenkins@brigade.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>