Update sphinx from 1.5.4 to 1.5.5

Author: pyup-bot

requirements/doc.txt

@@ -1,7 +1,7 @@
 -r ./base.txt
 docutils==0.13.1
 requests[security]==2.13.0
-sphinx==1.5.4
+sphinx==1.5.5
 reportlab==3.4.0
 alabaster==0.7.10
 releases==1.3.0 # Updated from 1.2.0