upgrade all lambdas to node 8.10

[mikestaub]

For some reason when we use node 8.10 our backend lambdas fail to work, and no errors are thrown.

https://github.com/mikestaub/slack-lunch-club/blob/master/backend/.babelrc.js#L18

When we run the code locally via npm run:dist it works, which is concerning. There must be something different between our local dev environment and the lambda runtime.