Adjust testing methods for optional builds

[parul-l]

The current implementation creates custom markers on tests, for example:

@pytest.mark.optional
@pytest.mark.postgres
def test_run(monkeypatch):
...

and then calls different pytest commands in the .travis.yml file, for example:

    elif [ $PYPI_BUILD = 'only_plotting' ]; then
        python -m pytest -m "not postgres"

A cleaner way to do this is to use pytest's skipif method:

@pytest.mark.skipif('rpy2' in sys.modules, reason="primrose[R] is optional")
def test_run():
...

This requires just one pytest command, regardless of which build is chosen (postgres, plotting, complete, etc.)