Correctly set RUBYLIB for JRuby.

When running
```
USE_JRUBY=1 bin/logstash agent -e 'input { pipe { command => "echo Hello World" }'
```
I would get the error:
```
Error opening script file: /Users/buckett/Documents/logstash/agent (No such file or directory)
```
adding ```set -x``` to ```bin/logstash``` you see the command getting executed is:
```
+ exec /usr/bin/java -Xmx500m -XX:+UseParNewGC -XX:+UseConcMarkSweepGC -Djava.awt.headless=true -XX:CMSInitiatingOccupancyFraction=75 -XX:+UseCMSInitiatingOccupancyOnly -jar /Users/buckett/Documents/logstash/vendor/jar/jruby-complete-1.7.9.jar -I /Users/buckett/Documents/logstash/lib/logstash/runner.rb agent -e 'input { pipe { command => "echo Hello World" }'
```
which is missing the RUBYLIB, this adds it back.