Add engines to package.json (#5585)

Based on this PR https://github.com/zeit/next.js/pull/5520 there should be `engines` definition in package.json as first warn.

Why i choose Node 8 as minimum? @timneutkens said (https://github.com/zeit/next.js/pull/5520/files#r228330327) that next.js should work on version 7.5 but automated tests in TravisCI are with versions 8 and 10. Version 7 was development branch. I think only production ready should be recommended.

package.json

@@ -67,5 +67,8 @@
     "taskr": "1.1.0",
     "wait-port": "0.2.2",
     "wd": "1.10.3"
+  },
+  "engines": {
+    "node": ">= 8.0.0"
   }
 }