servo: Merge #19591 - Actually run Taskcluster jobs on PRs (from aneeshusa:actually-trigger-taskcluster); r=edunham

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

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- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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Source-Repo: https://github.com/servo/servo
Source-Revision: 6440f17835e35069eddab88067f328a1a99d5376

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : e2c5882be224caa2999ab7b5bf8fbb6b98329385

servo/.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