Install Python dependencies using Homebrew Python

With recent changes to pip 1.5, insecure external URLs need an extra `--allow-external` flag to be installed. This change directly affects the OMERO dependencies especially PIL and caused the Jenkins job to be unstable in the last couple of days.

This commit provides a workaround  to this situation by using Homebrew pip to install the Python dependencies. Homebrew pip version is still 1.4 and should install PIL without problem while virtualenv created using Home-brew `virtualenv` install pip-1.5. This PR is expected to restore OMERO installation via Homebrew and pip 1.4 to normal for the imminent 4.4.10 release. To test it, check the OMERO-homebrew job turns green again.

More generally, for OMERO 5.0 and later, we may want to consider replacing the dependency on PIL by a dependency on [Pillow](http://pillow.readthedocs.org/en/latest/) which is a maintained fork of PIL.

/cc @will-moore, @aleksandra-tarkowska