title: I get `No module named _multiarray_umath` when using matplotlib
body: <p>When I run my tests in CI, I get the following error:</p>
  
  <pre><code>ImportError while importing test module '/home/tests/test_process.py'.
  Hint: make sure your test modules/packages have valid Python names.
  Traceback:
  ...
  .tox/py27/lib/python2.7/site-packages/matplotlib/pyplot.py:31: in &lt;module&gt;
      import matplotlib.colorbar
  .tox/py27/lib/python2.7/site-packages/matplotlib/colorbar.py:36: in &lt;module&gt;
      import matplotlib.contour as contour
  .tox/py27/lib/python2.7/site-packages/matplotlib/contour.py:14: in &lt;module&gt;
      import matplotlib._contour as _contour
  E   ImportError: numpy.core.multiarray failed to import
  ----- Captured stderr -----
  ImportError: No module named _multiarray_umath
  </code></pre>
  
  <p>What's going on here? I haven't made any changes to my code, but all the sudden
  my build started failing.</p>