supervisord fails for python dokku apps

For dokku python apps, virtualenv is used to setup a new python and module paths. The new python may be a different version than apt-get's supervisord (which has a hardcoded shebang to `/usr/bin/python`) which will result in the error: `ImportError: cannot import name MAXREPEAT`. Also, virtualenv's python module paths does not include the files required for apt-get installed supervisord, so execution will fail. 

The proposed solution is to modify `post-commit` to NOT setup the app's environment when executing supervisord. Instead, each command is modified to pass through `/exec` so that the correct environment is set.

I've attached a pull request. Thanks!