Assert Bazel version from tf_workspace() (#7799)

* Assert Bazel version from tf_workspace()

This ensures that projects depending on TensorFlow, e.g. TFServing, will
benefit from the version check.

The mandatory version has also been bumped up to Bazel 0.4.4 which has been out
for 21 days. It includes numerous very important fixes. TensorFlow also appears
to not work without it.

* update version to 0.4.5