Cloud foundry compatibility

This will make the build pack compatible with Cloud foundry

Small caveat for Cloud Foundry users:
You will need to use the following absolute path to erl in your procfile: /app/otp/bin/erl, ex:
```yaml
web: /app/otp/bin/erl -pa ebin deps/*/ebin -noshell -noinput -s myapp
```

_/app/opt/bin is in the PATH but for some reason it's not picked up by CF..._