Invoke jruby compilation from ruby code, not via jrubyc

This prevents 'java -jar jruby.jar -S jrubyc' from running something
from $PATH which breaks because jruby expects ruby code and rvm provides
a bash script in its place.
Merge pull request #811 from logstash/better-jrubyc-invocation

Invoke jruby compilation from ruby code, not via jrubyc