CI for `audio:summary_test` broken on tf2.0+py3

Reported by @jameswex (thanks!); seems to be affecting all PRs that have
a dependency path to this target.

The //tensorboard/plugins/audio:summary_test test target is
consistently timing out after 300s on only the Python 3 + TensorFlow 2.0
preview build configuration. Sample log:

https://travis-ci.com/tensorflow/tensorboard/jobs/198991418

Excerpt:

[ RUN      ] SummaryV1OpTest.test_audio_count_less_than_max_outputs
[  SKIPPED ] SummaryV1OpTest.test_audio_count_less_than_max_outputs
[ RUN      ] SummaryV1OpTest.test_audio_count_when_more_than_max
[  SKIPPED ] SummaryV1OpTest.test_audio_count_when_more_than_max
--- %< --- snip: more test cases --- %< ---
[ RUN      ] SummaryV1PbTest.test_wav_format_roundtrip
[  SKIPPED ] SummaryV1PbTest.test_wav_format_roundtrip
[ RUN      ] SummaryV2OpTest.test_audio_count_less_than_max_outputs
W0509 19:41:36.559446 140250432620352 deprecation.py:323] From /home/travis/virtualenv/python3.6.3/lib/python3.6/site-packages/tensorflow/python/summary/summary_iterator.py:68: tf_record_iterator (from tensorflow.python.lib.io.tf_record) is deprecated and will be removed in a future version.
Instructions for updating:
Use eager execution and: 
`tf.data.TFRecordDataset(path)`
Terminated

That test case:

https://github.com/tensorflow/tensorboard/blob/c23b5652812aa4efbdbe20a4be1ddd6bed624bec/tensorboard/plugins/audio/summary_test.py#L120-L126

I cannot reproduce locally, using:

• Bazel 0.22 (with --cache_test_results=no)
• Python 3.6.5
• tf-nightly-2.0-preview==2.0.0.dev20190509
• master, or commit cf6a390349f504df08975d32e58f183fdbbbf0c0 (as in
  the above build log)

I have not investigated.

Start with tensorboard/plugins/audio/summary_test.py lines 120-126 and the Travis job log for the Python 3 + TensorFlow 2.0 preview configuration. Run //tensorboard/plugins/audio:summary_test with the listed Bazel and dependency versions, then compare it with the passing configurations; done means the affected CI test no longer times out.