Fix Homebrew uninstallation

The last OMERO-homebrew-{stable, develop} jobs have been failing fail when calling `bin/brew doctor` with a
```
Warning: You have uncommitted modifications to Homebrew
```

This PR corrects this by using ``rm -rf $BREW_DIR/Cellar $BREW_DIR/.git && $BREW_DIR/bin/brew cleanup`` as suggested in the Homebrew installation scripts.

Additionally, this PR fixes the usage of virtualenv by extracting it under `$BREW_DIR/virtualenv` to ease its removal.