Prepare 1.9.1 release

[kevinykuo]

• The abalone custom estimator example has been removed from the tensorflow repo so I change the link to point to the file from a previous release. We should consider rewriting the tutorial to reflect https://www.tensorflow.org/guide/custom_estimators and https://github.com/tensorflow/models/blob/master/samples/core/get_started/custom_estimator.py.
• Added TF 1.11 to test matrix.
• Enable warnings_are_errors in travis.