Update sphinx from 1.7.8 to 1.8.0

Author: pyup-bot

requirements_dev.txt

@@ -5,6 +5,6 @@ watchdog==0.9.0
 flake8==3.5.0
 tox==3.2.1
 coverage==4.5.1
-Sphinx==1.7.8
+Sphinx==1.8.0
 twine==1.11.0
 sphinx_rtd_theme==0.4.1