Fix dependency on tensorflow version 1.4..0

[altanai]

Dependency on Tensorflow v1.4.0 causes the project to stop since it's hardcoded

if tf.__version__ != '1.4.0':
  raise ImportError('Please upgrade your tensorflow installation to v1.4.0!')

Can be corrected with

from setuptools._distutils.version import StrictVersion 

....

if StrictVersion(tf.__version__) < StrictVersion('1.4.0'):
    raise ImportError('Please upgrade your tensorflow installation to v1.4.* or later!')

I'd be happy to raise a PR for this and a few other small "good to have" changes if that's ok.
PS : awesome project :)