Require Sphinx 1.3

This is equal to the version used by Travis. 1.5-alpha is not working on
platform.sh because of an incompatible requests package version.

Author: wouterj

.platform.app.yaml

@@ -46,7 +46,7 @@ disk: 512
 # Build time dependencies.
 dependencies:
   python:
-    sphinx: ">=1"
+    sphinx: "~=1.3"
 
 # The hooks that will be performed when the package is deployed.
 hooks: