fix PATH in nats_stream_forwarder_ctl

After updated my devbox to v147, there is err showed in "sys/log/nats_stream_forwarder.stderr.log" said: "/var/vcap/jobs/nats/bin/nats_stream_forwarder_ctl: line 25: exec: bundle: not found"

We need add /var/vcap/packages/ruby/bin when export PATH, 

And Dr Nic said:
There should never be a reason for the following two paths to both be in $PATH
* /var/vcap/bosh/bin
* /var/vcap/packages/ANYTHING/bin

please ref the discuss in:
https://groups.google.com/a/cloudfoundry.org/forum/#!topic/vcap-dev/jB-YlxlzEFI