Node 6 support.
Github code search reports that there is no use of `async` or `await` in koa itself. Therefore, it should work fine in Node 6 (and anecdotally it seems to). Therefore I am struggling to understand the engine restriction on >=7.6 (sorry if I've missed something obvious). Obviously middleware authors can require Node > 7.6 if they are going to have `async` and `await` directly in `npm`-published code, but it would be great if Koa itself could relax this restriction for the core if it is unnecessary. My own reasoning is that I'll be damned if I put non-LTS versions of Node into production.