Pass allow-external to pip install to allow PIL installation

With recent changes to pip 1.5, insecure external URLs need an extra flag to be installed. This commit fixes the python_deps.sh script by setting the `--allow-external` flag. 

Alternatively, the dependency on PIL could be replaced by a dependency on [Pillow](http://pillow.readthedocs.org/en/latest/) which is a maintained fork of PIL.

To test this PR, check the OMERO-homebrew job turns green again.

/cc @will-moore, @aleksandra-tarkowska