Require let or const instead of var (no-var)

Fixes: standard/standard#633

Author: feross

eslintrc.json

@@ -25,6 +25,7 @@
   },
 
   "rules": {
+    "no-var": "error",
     "accessor-pairs": ["error", { "setWithoutGet": true, "enforceForClassMembers": true }],
     "array-bracket-spacing": ["error", "never"],
     "array-callback-return": ["error", {