Fix import errors during production docker build

Ghost User
2018-03-16T14:48:42+01:00

Together with !759 this fixes the import errors encountered during the production build. Using string formatting we fix the error of `Can't convert '_MockObject' object to str implicitly` The use of multiprocessing for the sphin...

Together with !759 (merged) this fixes the import errors encountered during the production build.

Using string formatting we fix the error of Can't convert '_MockObject' object to str implicitly

The use of multiprocessing for the sphinx builds raised EOFerror during the build.

waiting for workers...
Exception occurred:
  File "/usr/local/lib/python3.5/multiprocessing/connection.py", line 383, in _recv
    raise EOFError
EOFError
The full traceback has been saved in /tmp/sphinx-err-6s_795t7.log, if you want to report the issue to the developers.
Please also report this if it was a user error, so that a better error message can be provided next time.
A bug report can be filed in the tracker at <https://github.com/sphinx-doc/sphinx/issues>. Thanks!

Closes #611 (closed)