To comply with PEP 396 - expose __version__

[lc0]

While testing a newly released 0.8.0 I figured out, that adanet does not expose the version as many python packages do or as PEP-396 recommends

Other examples:

• TFhub - https://github.com/tensorflow/hub/blob/master/tensorflow_hub/__init__.py#L48
• tfx - https://github.com/tensorflow/tfx/blob/master/tfx/__init__.py#L18
• addons - https://github.com/tensorflow/addons/blob/master/tensorflow_addons/__init__.py#L32
• tensorboard - https://github.com/tensorflow/tensorboard/blob/master/tensorboard/__init__.py#L99

[lc0]

cc @EugenHotaj @cweill

[cweill]

@lc0 Thanks for the PR, we'll merge it in.