Force master doc to 'index' for Sphinx

not sure why this is necessary for buildbot but not local dev..

docs/conf.py

@@ -15,6 +15,11 @@
 # sys.path.insert(0, os.path.abspath('.'))
 
 
+# TODO: not sure why i had to explicitly declare this for the sake of buildbot,
+# when it worked for me fine locally without doing this.
+master_doc = 'index'
+
+
 # -- Project information -----------------------------------------------------
 
 project = 'Messkit'