Increase max run time to 2 hours

Based on the Taskcluster log for
(https://taskcluster-artifacts.net/HkxDh1oKQceXH1fL9RSGow/0/public/logs/live_backing.log),
it looks like the job is running successfully but hitting the max time limit.

Our builds + tests shouldn't take this long to run normally,
but we'll need to set up sccache with cache uploading/downloading
in order to improve most build times.

For now, bump the max run time to 2 hours;
since we complete one build + som tests within an hour,
2 hours should be enough for the second build to finish as well.

.taskcluster.yml

@@ -16,7 +16,7 @@ tasks:
           - pull_request.reopened
           - pull_request.synchronize
     payload:
-      maxRunTime: 3600
+      maxRunTime: 7200
       image: 'servobrowser/servo-linux-dev:servo-linux-build-deps-2017-06-30'
       command:
         - /bin/bash