Update thumbor-worker Upstart job

Removed enabled=1 check, as you can just use `update-rc.d enable|disable` for that purpose (or add `exit 0` to the default file). Changed instance variable to be more meaningful. Removed bad usage of /tmp (it was unsafe and allowed escalation to root).
Updated thumbor upstart job

Removed check of enabled=1 (see previous commit), renamed port var to ports, adjusted start/stop events slightly.
Changed port to PORT, since default files will override the variable
ports went back to port, so default files do not break
Merge pull request #349 from CameronNemo/master

Update Upstart jobs