Fixes this error:
```
FAIL: test_setup_command.test_build_sphinx_return_nonzero_status
----------------------------------------------------------------------
Traceback (most recent call last):
File "/nix/store/zfmk6mqmr1046bh0cnh06frd0bb0nr20-python2.7-nose-1.3.7/lib/python2.7/site-packages/nose/case.py", line 197, in runTest
self.test(*self.arg)
File "/tmp/nix-build-python2.7-Sphinx-1.3.6.drv-0/Sphinx-1.3.6/tests/test_setup_command.py", line 55, in deco
func(pkgrootdir, proc)
File "/tmp/nix-build-python2.7-Sphinx-1.3.6.drv-0/Sphinx-1.3.6/tests/test_setup_command.py", line 110, in test_build_sphinx_return_nonzero_status
assert proc.returncode != 0, 'expect non-zero status for setup.py'
AssertionError: expect non-zero status for setup.py
```
Tested on Linux
- python 2.7
- python 3.5
e487772722