fix platform.sh build

Author: robfrawley

.platform.app.yaml

@@ -46,10 +46,12 @@ disk: 512
 # Build time dependencies.
 dependencies:
   python:
-    sphinx: "1.3.6"
+    virtualenv: 15.1.0
 
 # The hooks that will be performed when the package is deployed.
 hooks:
     build: |
-      pip install git+https://github.com/fabpot/sphinx-php.git
-      make -C _build html
+      virtualenv venv
+      source venv/bin/activate
+      pip install --ignore-installed git+https://github.com/fabpot/sphinx-php.git
+      SPHINXBUILD=../venv/bin/sphinx-build make -e -C _build html