Update sphinx from 1.5.5 to 1.5.6 (#2471)

requirements/docs.txt

@@ -6,7 +6,7 @@
 # Docs, we could omit the Sphinx entry since they provide it, however that
 # would not help those who wish to build the docs locally.
 
-Sphinx==1.5.5
+Sphinx==1.5.6
 sphinx-autobuild==0.6.0
 sphinx-rtd-theme==0.2.4
 sphinxcontrib-httpdomain==1.5.0