also build on JRuby

Because I want to integrate this gem into a [Maven plugin](https://github.com/GeoDienstenCentrum/sass-maven-plugin) I want to be sure that it will run/build on JRuby. 

I thought about extending the build matrix with various JDK implementations, but that seems unnecessary overhead ATM.

see GeoDienstenCentrum/sass-maven-plugin#3