Switch to official images in readme

Signed-off-by: Ben Firshman <ben@firshman.co.uk>

README.md

@@ -8,7 +8,7 @@ Fast, isolated development environments using Docker.
 
 Define your app's environment with Docker so it can be reproduced anywhere:
 
-    FROM orchardup/python:2.7
+    FROM python:2.7
     ADD . /code
     WORKDIR /code
     RUN pip install -r requirements.txt
@@ -25,7 +25,7 @@ web:
    - "8000:8000"
    - "49100:22"
 db:
-  image: orchardup/postgresql
+  image: postgres
 ```
 
 (No more installing Postgres on your laptop!)