Rasa-nlu-contrib

- Fix issue with `server` dependency

- Minimize the bare installation
* server (HTTP) can be installed now separately
- Rasa-NLU version 0.15.1
- Known issues:
* Tests
+ tests/base/test_extractors.py :: 1 failing tests due Python version
+ tests/base/test_featurizers.py :: 4 failing tests due Python version
+ tests/base/test_multitenancy.py :: 5 failing tests due Python version
+ tests/training/test_train.py :: 5 failing tests due Python version
* Requirements
+ `tox` is not used due to dependecy issue on `six`

- Initial release
- Rasa-NLU version 0.15.1
- Known issues:
* tests/base/test_extractors.py :: 1 failing tests due Python version
* tests/base/test_featurizers.py :: 4 failing tests due Python version
* tests/base/test_multitenancy.py :: 5 failing tests due Python version
* tests/training/test_train.py :: 5 failing tests due Python version