Move python dependencies inside a virtualenv

Author: damz

.platform.app.yaml

@@ -46,10 +46,14 @@ 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
+      virtualenv .virtualenv
+      . .virtualenv/bin/activate
+      export PIP_USER=
+      pip install --upgrade pip wheel
+      pip install sphinx git+https://github.com/fabpot/sphinx-php.git
       make -C _build html