Enable "no-var" ESLint rule

To ensure consistency going forwards, enforce usage of const/let

.eslintrc

@@ -8,6 +8,7 @@
   ],
   "rules": {
     "mocha/no-exclusive-tests": "error",
+    "no-var": "error",
     "indent": ["error", 2, {
       "ArrayExpression": "first",
       "ObjectExpression": "first"