chore(package): update eslint to version 3.15.0

https://greenkeeper.io/

Author: greenkeeperio-bot

package.json

@@ -39,7 +39,7 @@
   },
   "devDependencies": {
     "coveralls": "^2.11.9",
-    "eslint": "^2.12.0",
+    "eslint": "^3.15.0",
     "istanbul": "^0.4.3",
     "mocha": "^2.5.3",
     "pre-commit": "^1.1.3",