Move to Node 10

[brendankenny]

It's that time again.

Node 10.13 was moved to LTS at the end of October 2018. Historically we've waited until some time in the new year to move over to the new version. Last time we did it with the split to 3.0.

There's not a ton of new stuff (for-await (including for readable streams), Promise.finally, BigInt, fs/promises, global URL, --experimental-worker), but they offer some pretty decent QoL improvements.

Regardless of timeline, we'll need to update our testing configs to start testing node 11 as well.