Checkout a stable release for testing.

TestDrive.md

@@ -18,6 +18,7 @@ This one-shot script clones the code, initialises Docker swarm mode and then dep
 # docker swarm init --advertise-addr eth0 && \
   git clone https://github.com/alexellis/faas && \
   cd faas && \
+  git checkout 0.5.6b-alpha && \
   ./deploy_stack.sh && \
   docker service ls
 ```