travis: add GitHub fallback for Bazel download (#1667)
travis: add GitHub fallback for Bazel download (#1667)

Summary:
As suggested by @nfelt in:
<https://github.com/tensorflow/tensorboard/pull/1661#pullrequestreview-181946869>

Test Plan:
Verify that the new chunk of shell script has the following properties:

  - When executed with the environment variables at the top of the
    Travis config, successfully downloads Bazel 0.16.1 and returns 0.
  - When executed with `BAZEL=0.19.2` (which does not exist in the
    mirror) and shasum updated per the instructions in the comment,
    successfully downloads Bazel 0.19.2 and returns 0.
  - When executed with `BAZEL=0.wat.0` (which does not exist), fails
    and returns 1.
  - When a bit is flipped in the shasum, fails and returns 1, having
    removed the invalid downloads.

Note that Shellcheck reports no errors or warnings with `#!/bin/sh`.

wchargin-branch: travis-bazel-download-github-fallback

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