v2.2.3

Koa version 2 is now tagged as latest on npm as native async / await has landed in node 7.6. This release brings us in line with the rest of the koa ecosystem on this.

• remove publishConfig tag so that koa-v2 publishes are tagged as latest
• set the minimum nodeEngine to 7.6 in package.json
• change install instructions to show next / koa-v2 tags not required on npm install
• update Travis config to test against node 7.6 and above