missing test dependencies

[michamos]

After installing in a clean environment, the following dependencies were missing to run the tests (besides those installed by pip install -e .: pytest, pytest-coverage, pytest-flake8.

[jacquerie]

You'll want to install them with pip install -e .[tests], like Travis does: https://github.com/inspirehep/inspire-dojson/blob/d22c0098a2539d035af48688064787bacc690363/.travis.yml#L43