This will allow platform changes to be fully rolled out to all active branches with recipes.

Note that the recipe /behavior/ should still be pinned, but this allows us to update client protocols (i.e. how recipe_engine talks to kitchen/remote_run and/or swarming, logdog, etc.), as well as do security patches (i.e. if we need to bump a version of git which has a different command line api), which will avoid issues like the original issue in crbug.com/731755

This will likely necessitate putting the dev/beta/stable branch information in luci_config too.