Use Homebrew Python rather than virtualenv

With recent changes brought to the main Homebrew repository, virtualenv Python
cannot be used by the Formula installer. To reduce maintenance burden, this
commit now installs Homebrew Python instead of a virtual environment. All
Python dependencies should be installed there during testing.
Fix scc installation
Create virtualenvironment (using Homebrew Python) for OMERO dependencies

- Use pip install virtualenv to install a virtual environment
- Activate the virtual environment only prior to running omero_python_deps
Merge pull request #1274 from sbesson/homebrew_python

Use Homebrew Python rather than virtualenv