Bump sphinx from 1.7.9 to 1.8.0

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 1.7.9 to 1.8.0.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/master/CHANGES)
- [Commits](sphinx-doc/sphinx@v1.7.9...v1.8.0)

Signed-off-by: dependabot[bot] <support@dependabot.com>

requirements.txt

@@ -12,5 +12,5 @@ gunicorn==19.9.0
 pycodestyle==2.4.0
 pydocstyle==2.1.1
 requests==2.19.1
-Sphinx==1.7.9
+Sphinx==1.8.0
 Werkzeug==0.14.1