Removes AIRBRAKE_API_KEY as required env var
Removes AIRBRAKE_API_KEY as required env var

https://github.com/thoughtbot/suspenders/commit/ca11a8638090d9f7b6af1ac55586d24d05c77df6
removed the dependency on `airbrake` in favor of `honeybadger`.

The `AIRBRAKE_API_KEY` was left in `app.json` but is no longer required.
This change removes it.

toobulkeh/suspenders,headwayio/suspenders,welaika/welaika-suspenders,Mossio/hashira-rails,welaika/welaika-suspenders,thoughtbot/suspenders,pacso/suspenders,headwayio/voyage,zamith/suspenders,pacso/suspenders,lorentrogers/suspenders,zamith/suspenders,Mossio/hashira-rails,lorentrogers/suspenders,delphaber/suspenders,mukkoo/welaika-suspenders,headwayio/voyage,zamith/suspenders,headwayio/voyage,Mossio/hashira-rails,lorentrogers/suspenders,delphaber/suspenders,thoughtbot/suspenders,toobulkeh/suspenders,headwayio/suspenders,toobulkeh/suspenders,headwayio/suspenders,delphaber/suspenders,thoughtbot/suspenders,pacso/suspenders,Mossio/hashira-rails,welaika/welaika-suspenders,mukkoo/welaika-suspenders,headwayio/suspenders,headwayio/voyage,mukkoo/welaika-suspenders