Fix $IMAGE variable.

In the current version of dokku the $IMAGE variable is no longer passed in to pluginhooks, causing this script to break. The fix is easy, just need to change it from being `IMAGE="$2"` to `IMAGE="app/$APP"`.

More details about the fix, and the commit that changed this, here:
https://github.com/progrium/dokku/issues/313#issuecomment-31210408