intermittent CI failure

[chriddyp]

https://circleci.com/gh/plotly/dash-renderer/154

======================================================================
FAIL: test_simple_callback (tests.test_render.Tests)
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Traceback (most recent call last):
  File "/home/ubuntu/dash-renderer/tests/test_render.py", line 461, in test_simple_callback
    len('hello world')
AssertionError: 11 != 12

[chriddyp]

See #36 (comment) for a discussion about a fix