Ignore eslint for greenkeeper updates. (#580)

As we don't plan to update to Node.js v4 any time soon,
we can't update eslint cause it requires Node.js => v4.

package.json

@@ -52,5 +52,8 @@
       "wrappers"
     ]
   },
+  "greenkeper": {
+    "ignore": ["eslint"]
+  },
   "license": "MIT"
 }