Make CheckRevisionStep Twisted-compatible

The inlineCallbacks decorator requires that the function it wraps
(here, the run() method of CheckRevisionStep) is a generator, so that
is is able to suspend execution to require it to insert callbacks.

Make run() into a generator by yielding the Deferred return value.

buildbot/master/files/config/factories.py

@@ -41,7 +41,7 @@ def run(self):
                 )
             )
 
-        defer.returnValue(SUCCESS)
+        yield defer.returnValue(SUCCESS)
 
 
 class ServoFactory(util.BuildFactory):