[TEST/CI] Add tests covering KerasModelArtifact

[parano]

Currently, there isn't any integration test covering the KerasModelArtifact:

• Similar to PR [CI] Include onnx artifact test on travis #726, add a unit test assuming keras and tensorflow pypi package installed, and create a BentoService with KerasModelArtifact, test the major functionalities including save, load, run inference
• Build a docker container with the saved bundle and test the API server endpoint, see related PR Add integration tests #855 and [CI] Refactor API Server integration tests #871 which have some utility function code that can be re-used for this
• Add a script ./travis/run-keras-tests.sh that runs pip install keras==... and pytest tests/test_xgboost_model_artifact.py ...
• Add a separate task in travis.yml that launches the test script https://github.com/bentoml/BentoML/blob/master/.travis.yml

[parano]

For @bojiang to review related tests, feel free to close if you think those tests are in good shape