README: Make call to pulp_redis explicit

Earlier today in `pulp/pulp_installer`
pulp/pulp_installer#332 got merged. This leads
to one having to specify the pulp_redis role explicitly.

README.md

@@ -61,6 +61,7 @@ The following is a simple quickstart for installing a local Galaxy server. It re
             version: 2.0.0b2
         pulp_api_workers: 4
       roles:
+        - pulp_redis
         - pulp_database
         - pulp_workers
         - pulp_resource_manager