ci: update Travis to Bazel 0.21.0 (#1767)
ci: update Travis to Bazel 0.21.0 (#1767)

Summary:
Testing on fresh Trusty VMs on GCP suggests that this may suffice to
stop the strange errors wherein Bazel invokes GCC with invalid flags,
though it should be noted that we do not actually understand the root
cause or the mechanism of failure.

Test Plan:
That this build should pass on Travis does not actually indicate that
this commit fixes the underlying problem, because the build will use
a new Bazel cache due to the version change. So: merge now, and hope
that this fixes the problem.

wchargin-branch: travis-bazel-0.21.0

tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard