Shrink Koa's support to Node.js' currently supported versions
I'm starting a new project today with Koa and I noticed it _"requires"_ Node.js v7.6 in order to work. Well, since the Node.js' 7th version is far gone - with the 10th one being the oldest still "active" (maintenance patches only) and the 12th being the current LTS, maybe it's time to update Koa's supported Node.js versions.

**Is there any good reason to not deprecate Koa's support for Node.js versions prior to the LTS one?**

PS: I see there's #1502 that went over this topic before, but it seemed the approach took by its author was a bit aggressive, and the matter didn't have a chance to be taken seriously.