build docs under Python 3.7

I got a weird failure in PR #2231 in Travis job
https://travis-ci.com/MDAnalysis/mdanalysis/jobs/189553940 :

   Exception occurred:
     File "/home/travis/miniconda/envs/test/lib/python3.5/site-packages/sphinx/ext/napoleon/docstring.py", line 703, in _parse_raises_section
       m = self._name_rgx.match(_type).groupdict()
   AttributeError: 'NoneType' object has no attribute 'groupdict'

and the docs build find locally (under Python 3.6)

.travis.yml

@@ -50,6 +50,7 @@ matrix:
     - env: NAME="Doc"
            MAIN_CMD="cd package && python setup.py"
            SETUP_CMD="build_sphinx"
+	   PYTHON_VERSION=3.7
            BUILD_DOCS=true
            BUILD_CMD="cd ${TRAVIS_BUILD_DIR}/package && python setup.py build_ext --inplace"
            INSTALL_HOLE="false"