[COOK-3415] Pass `--pre` to install supervisor

pip 1.4 now strictly checks the version of packages and if it does not follow PEP426 it will be assumed to be pre-release. In this case supervisor is following it but all version have 'a' or 'b' in it, and those are considered to be alpha and beta respectively.

This can be fixed by passing '--pre' to the `python_pip` resource.

http://tickets.opscode.com/browse/COOK-3415