Remove Tensorflow dependency to support Python 3.8

[Tomcli]

Tensorflow 1.14 is no longer in Python 3.8 library. Therefore anyone who wants to install aix360 on Python 3.8 will have this dependency error. Since we don't have a plan to migrate Tensorflow version, we should at least remove the Tensorflow dependency so that other open source project can still use the official AIX package.

[vijay-arya]

@Tomcli pull #174 addresses this.

[Tomcli]

Thanks @vijay-arya , can you also cut a new release on Pypi so we can use it via pip install? Thanks.