Check that a redis instance actually exists

The pre-release hook causes containers without matching redis containers to always get an environment variable that looks like: REDIS_URL=redis://:6379 set. This is a problem if you wish to share one redis instance between two containers, or name your redis instance with a name that doesn't match the app name, and later link it manually.
Merge pull request #22 from BradWhittington/patch-1

Check that a redis instance actually exists