accesslint-bot should not comment on closed PR's

[tysongach]

Scenario

With a typical rebase workflow, it is likely that accesslint-bot will comment on closed/merged PR’s, which can be annoying/noisy (obviously not a show-stopper).

1. I open a PR.
2. accesslint-bot comments with a few issues.
3. The PR sits stays open for a bit; others on the team are merging to master
4. I rebase off master locally, push --force to GitHub to update the PR with the rebase, then immediately merge to master, and push master.
5. While step 4 is in progress, AccessLint is running on CI, and finally comes back to comment on the PR, but it's already been closed/merged.

Here’s what that looks like in a PR: thoughtbot/bourbon.io#61

[tysongach]

Moved to Trello: https://trello.com/b/PpFdXtix/accesslint (private link)