Fix for LOGSTASH-1983

First crack at a fix for LOGSTASH-1983

Two possible issues:

1 - This might break dev using ruby, since we'd be introducing an extra variable. 
2 - If someone does an inplace upgrade with an extra version of jruby getting the jar file will break. Maybe check for more then one item, and error? or find a more elegant way to pick the jar file. 

For the record, can't pass the * into the exec as it won't be expanded correctly.