Bug 1194767 - use slugid 1.0.6 in ./mach taskcluster-graph command. r=wcosta

This included adding the slugid 1.0.6 python source code in /python since slugid
is now a dependency of the ./mach taskcluster-graph command, as well us updating
references that used it. Previously the implementation was in-tree.

testing/taskcluster/mach_commands.py

@@ -277,7 +277,7 @@ class Graph(object):
         help="Run the tasks with the interactive feature enabled")
     def create_graph(self, **params):
         from taskcluster_graph.commit_parser import parse_commit
-        from taskcluster_graph.slugid import slugid
+        from slugid import nice as slugid
         from taskcluster_graph.from_now import (
             json_time_from_now,
             current_json_time,