text: Title: Rewrite semver tests to use pytest's parametrize functionality
  Body: via https://github.com/jupyter/repo2docker/pull/613#discussion_r263836315:
  
  > In a future refactor, we should use pytest's parameterize capability to feed in versions to get closer to DRY. [See example from JupyterHub tests](https://github.com/jupyterhub/jupyterhub/blob/master/jupyterhub/tests/test_proxy.py#L169).
  
  This would make a good issue for getting started on contributing to repo2docker and/or someone who wants to learn more about pytest.