Use 'esnext' JSHint config setting for compatibility with JSHint v2.8.0

Hound CI is currently using JSHint v2.8.0 which
uses 'es3', 'es5' and 'esnext' config options instead of
the 'esversion' option in v2.9.0 and later.

.jshintrc

@@ -10,6 +10,7 @@
   "undef": true,
   "unused": true,
   "esversion": 6,
+  "esnext": true,
   "globals": {
     "chrome": false,
     "h": false,