ci: remove node 8 tests, add node 14 tests

BREAKING CHANGE: drop official support for Node 8

fix: remove undocumented legacy callbacks (probot#18)

`context.done` is from the pre-node4.10 runtime days of lambda, it was deprecated in favor of callback, which is optionally sidelined in favor of an async/promise-based return. If the function receives no callback and returns a promise or is marked as async, you should simply return/throw in order to actuate the callbacks.

fix(serverless): gracefully fail with 400 for null body (probot#38)

fix: make package compatible with probot v10 (probot#46)

0.5.0

Merge pull request probot#30 from probot/npm-audit-fix

Further update deps, cut 0.4.0

chore: bump dependencies