Promote no-undef to be treated as an error (#669)

Fixes #498

Author: gaearon

config/eslint.js

@@ -126,7 +126,7 @@ module.exports = {
     'no-template-curly-in-string': 'warn',
     'no-this-before-super': 'warn',
     'no-throw-literal': 'warn',
-    'no-undef': 'warn',
+    'no-undef': 'error',
     'no-unexpected-multiline': 'warn',
     'no-unreachable': 'warn',
     'no-unused-expressions': 'warn',