Use modified docker-py to mask change in Docker error message

Before this change, when using Docker 17.06 we get:
tests/integration/container_test.py:121: in test_create_with_restart_policy
    'You cannot remove a running container', err
    E   AssertionError: 'You cannot remove a running container' not found in u'You cannot remove a restarting container

The modified image simply checks for "You cannot remove a".

test/600_proxy_docker_py.sh

@@ -3,7 +3,7 @@
 
 . "$(dirname "$0")/config.sh"
 
-IMAGE="joffrey/docker-py:1.9.0-rc2"
+IMAGE="weaveworks/docker-py:1.9.0-prime"
 
 docker_py_test() {
     SHARD=$1