Moved the requirements file

Author: javiereguiluz

.platform.app.yaml

@@ -56,5 +56,5 @@ hooks:
       # Platform.sh currently sets PIP_USER=1.
       export PIP_USER=
       pip install pip==9.0.1 wheel==0.29.0
-      pip install -r requirements.txt
+      pip install -r _build/.requirements.txt
       make -C _build html