Fix Homebrew uninstallation and move virtualenv/ to support folder

The last OMERO-homebrew-{stable,develop} jobs have been fail when calling
`bin/brew doctor` with a "Warning: You have uncommitted modifications to
Homebrew". This PR applies the uninstallation mechanism suggested in the
Homebrew installation script.
Additionally, a VENV_DIR variable is defined to extract virtualenv into a
proper support.