Add support for PR updates

[phillipj]

Currently the bot only responds to created PR's. Adding support for reporting Travis build status on PR changes (new or forced commits) would be great.

[phillipj]

As the webhooks have currently been configured to "send all events", the bot already gets notified about updates, it just ignores them...

https://github.com/nodejs-github-bot/github-bot/blob/master/server.js#L15 would be particularly interresting

[MylesBorins]

I'd be up for taking this on if you want to assign it to me. Can potentially hack ion it from 10k feet this afternoon 😄

[phillipj]

@Fishrock123 did you fix updates with your last changes in #15?

[Fishrock123]

@phillipj Oh, yes actually: https://github.com/nodejs-github-bot/github-bot/blob/master/scripts/display-travis-status.js#L9-L10

[Fishrock123]

Note: I have not tested this with the PR-commenting, only the PR status updates.