Upgrade to Probot v10

[gr2m]

Hello Lambda friends,

we just released Probot v10 and I'd like to upgrade @probot/serverless-lambda for compatibility. Would any existing user/contributor would like to collaborate on that with me?

More importantly, I'd like to make this package obsolete with Probot v11. I'm not sure how it would look like, but making Probot play nicely with serverless environments is our priority for the next release. Please join the discussion at
probot/probot#1286

[soberstadt]

I would love to see this happen too! I know two things that need to get updated are the imports in index.js:
https://github.com/probot/serverless-lambda/blob/52dafe0b84b117b97570b7c8add96552ff6e331b/index.js#L2-L3
to

const { resolve } = require('probot/lib/helpers/resolve-app-function')
const { findPrivateKey } = require('probot/lib/helpers/get-private-key')

[scolandrea]

Thanks, @soberstadt I just created a PR based on your recommendations :)