Bug 1237739 - Sort keys in generated graph; r=garndt

Deterministic output is easier to read and test. We're not fully
deterministic with this patch, but it's a start.

testing/taskcluster/mach_commands.py

@@ -570,7 +570,7 @@ class Graph(object):
             graph.pop('scopes', None)
             graph.pop('metadata', None)
 
-        print(json.dumps(graph, indent=4))
+        print(json.dumps(graph, indent=4, sort_keys=True))
 
 @CommandProvider
 class CIBuild(object):