Support Python 3 for tensorflow-io

[yongtang]

At the moment, tensorflow-io package could be built within docker image tensorflow:custom-op. By default tensorflow:custom-op uses python 2.7.6. The package has not been built and tested with python 3. We should support python 3.

Also, from tensorflow repo there is a lot of interests for python 3.7 support, so 3.7 should be considered as well.

[yongtang]

From PR #17, Python 3.4/3.5/3.6 are supported now. Python 3.7 is not supported yet, as Travis CI's trusty does not support Python 3.7.

[yongtang]

We still don't have 3.7 on Travis CI yet. Though tensorflow is not supporting 3.7 anyway, and we already covered 3.4-3.6 on Ubuntu 14.04-18.04. I think this issue could be closed.