mongo:link breaks apps without a mongo database

When you deploy an app without an existing mongo database to link to, it will fail and halt the deploy process. Make sure the database exists before trying to modify the app container.
Prevent infinite loop

config:set restarts the release step, which will re-run the pre-release hooks and thus also the mongo:link call. This creates an infinite loop of appending to the ENV file!
Fix syntax errors...oops. >.>
I keep missing stuff...
Merge pull request #1 from stephen-fanfair/patch-1

mongo:link breaks apps without a mongo database