Fix CI on TensorFlow

[taehoonlee]

This PR is a follow-up to #10674. Is there a difference between build environments for PRs and the master branch? :( There was no problem in #10674 but the recent CI on the master branch have been failing.

[taehoonlee]

@fchollet, I tried the following (A: data_utils_test.py, B: test_multiprocessing.py):

• Enabling A, Enabling B with WORKERS=2 (success on Update TensorFlow as 1.9 on Travis #10674, but fail on the master)
• Enabling A, Enabling B with WORKERS=1 (fail on this PR)
• Enabling A, Disabling B (fail on this PR)
• Disabling A, Enabling B with WORKERS=2 (fail on this PR)
• Disabling A, Disabling B (success on this PR, but on the master..?)

If the problem persists after this PR is merged, we may need to revert TensorFlow 1.7 again.

[taehoonlee]

Please note that the test coverage rate on TF drops by 1.2-1.3% if the two tests are skipped.