ci: pin to non-broken estimator nightlies (#2668)
ci: pin to non-broken estimator nightlies (#2668)

Summary:
Due to <https://github.com/tensorflow/tensorflow/issues/32621>.

This unblocks our CI until tomorrow’s nightlies are pushed.

Test Plan:
Ran this 5-line shell command (with `pip install` changed to `echo) once
with `TF_VERSION_ID` set to each value from the `env` section, and noted
that in all cases it succeeds and that it only prints output in the
correct case.

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