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.

.taskcluster.yml

@@ -11,7 +11,10 @@ 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
@@ -31,4 +34,4 @@ tasks:
       description: Run Linux tests.
       owner: '{{ event.head.user.email }}'
       source: '{{ event.head.repo.url }}'
-
+allowPullRequests: public