GH-177: Update tests

[tabergma]

• Get rid of TRAVIS env
• Per default only "fast" tests are executed when running pytest
• To run all test you have to execute pytest --runslow
• Update all tests to use pathlib.

closes #177

[alanakbik]

Do the integration tests really need to be skipped on travis? They only make small models and are important for catching lots of errors.

[alanakbik]

Perhaps remove @pytest.mark.slow from the integration tests? Its good to skip the tests for loading all the models and embeddings (i.e. test_embeddings.py) since they are too big, but the training, loading and saving steps should be tested in different variations since they caused lots of errors in the last release.

[tabergma]

Update: Travis runs unit and integration test on a pull request. On a normal push just the unit tests are executed. Slow tests (such as test_embeddings.py) are not executed at all on Travis.

To run only unit tests: pytest
To run unit + integration tests: pytest --runintegration
To run unit + slow tests: pytest --runslow

[alanakbik]

Great work, thanks!