Homebrew installation: virtualenv URL (rebased onto develop)



This is the same as gh-854 but rebased onto develop.

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This PR should fix an unbound variable error in [OMERO-homebrew-stable#134](http://hudson.openmicroscopy.org.uk/job/OMERO-homebrew-stable/OSX=10.7.5/134/console).

- initialise VENV_URL to the list of variables
- use pip installation instead of Homebrew installation for sec