ci: unpin 2019-09-17 estimator nightlies (#2835)
ci: unpin 2019-09-17 estimator nightlies (#2835)

Summary:
This reverts commit e4a70498e833ba50a008db20a48f1444abb2fa84.

Test Plan:
CI suffices.

wchargin-branch: unpin-estimator-20190917-nightly
tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard