Don't run CI on PR closure

[marco-c]

See also marco-c/taskcluster_yml_validator#31.

[spattk]

Hi @marco-c , as this a good-first-bug, can I pick this up?

[marco-c]

Sure, feel free! You have an example here: mozilla/microannotate@6e21417.

[spattk]

Sure, thanks. Please review the PR
#505

[marco-c]

Fixed by #505.