Switches to use venv instead of virtualenv for tests since we use venv everywhere else.
Added environment variable 'TEST_USE_VENV' to select venv over virtualenv.
Renamed TEST_USE_VENV to ENSTALLER_TEST_USE_VENV for better scoping.
Merge pull request #115 from enthought/use_venv_for_tests

Changes unit tests to use venv instead of virtualenv