Add bin/releases file

Encountered this error while futzing with this build pack:

```
...
-----> Fetching custom git buildpack... done
-----> Multipack app detected
=====> Downloading Buildpack: https://github.com/gregburek/heroku-buildpack-runit.git
chmod: cannot access `/tmp/buildpackXgaUN/bin/release': No such file or directory

 !     Push rejected, failed to compile Multipack app

...
```

This PR adds a nominally blank releases file, which allows the build to succeed.