Bugs?

Hi there,

I've had some troubles setting up redis with this plugin.

* There is a `cat` in the `commands` file, which caused deployment for ALL apps to break (no output on client, no successful deployment)
* The pre-release hook screws up the ENV file of every app with empty/broken redis config values like `export REDIS_IP=` and `export REDIS_URL=redis://:6379`. This becomes a real problem when you link an app to a redis container with a different name, because after each deployment, your correct variables are overwritten.

Please don't merge this PR, I have no idea if the trailing `cat` has had some purpose and I'm aware removing the hook will break stuff for other users, this is just what I had to do to get it working for me. Maybe you can `exit` in https://github.com/Foolaround/dokku-redis-plugin/blob/84f03f88e6aa2a2e1378277d7c76b40781514847/commands#L99 if no matching redis container can be found?