Broken imports on 0.5.0.2 release

[adriangonz]

What's failing?

On the latest release, there are some missing files. In particular, the pre-compiled tensorflow protobuffs introduced in #942 are not included. This breaks some import paths when tensorflow is not present.

For instance,

>>> from seldon_core import utils
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/agm/.virtualenvs/fresh/lib/python3.6/site-packages/seldon_core/utils.py", line 10, in <module>
    from seldon_core.proto import prediction_pb2
  File "/home/agm/.virtualenvs/fresh/lib/python3.6/site-packages/seldon_core/proto/prediction_pb2.py", line 17, in <module>
    from seldon_core.proto.tensorflow.core.framework import tensor_pb2 as tensorflow_dot_core_dot_framework_dot_tensor__pb2
ModuleNotFoundError: No module named 'seldon_core.proto.tensorflow'

Suggested fix

We need to add the missing files to the MANIFEST.in file, so that they get included in the sdist release. To avoid this happening again in the future, we can introduce tox, which will wrap our tests and then run them in a fresh new environment each time (created from the setup.py manifest).

[gsunner]

0.5.0.3 release should now fix this