Auto merge of #19591 - aneeshusa:actually-trigger-taskcluster, r=edunham

Actually run Taskcluster jobs on PRs

Spawn TC jobs on the following events:
  - Pull request opened or reopened
  - New commit made in pull request

Spawn TC jobs for pull requests started by anybody,
not just trusted collaborators.
This is safe for now as we do not provide any scopes to spawned jobs.

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- [ ] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

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.taskcluster.yml

@@ -11,10 +11,13 @@ tasks:
     workerType: '{{ taskcluster.docker.workerType }}'
     extra:
       github:
-        events: []
+        events:
+          - pull_request.opened
+          - pull_request.reopened
+          - pull_request.synchronize
     payload:
       maxRunTime: 3600
-      image: servobrowser/servo-linux-dev
+      image: 'servobrowser/servo-linux-dev:servo-linux-build-deps-2017-06-30'
       command:
         - /bin/bash
         - '--login'
@@ -31,4 +34,4 @@ tasks:
       description: Run Linux tests.
       owner: '{{ event.head.user.email }}'
       source: '{{ event.head.repo.url }}'
-
+allowPullRequests: public