fix(engines): update engines.node in package.json

Not a major version bump because the lib never worked with node 6 anyway.

Closes testing-library#110

Author: Kent C. Dodds

package.json

@@ -5,7 +5,7 @@
   "main": "dist/index.js",
   "typings": "typings/index.d.ts",
   "engines": {
-    "node": ">=6"
+    "node": ">=8"
   },
   "scripts": {
     "add-contributor": "kcd-scripts contributors add",