chore(package): update eslint to version 3.15.0 (#988)

https://greenkeeper.io/

Author: greenkeeper[bot]

package.json

@@ -30,7 +30,7 @@
   "devDependencies": {
     "benchmark": "~2.1.2",
     "bufferutil": "~2.0.0",
-    "eslint": "~3.14.0",
+    "eslint": "~3.15.0",
     "eslint-config-semistandard": "~7.0.0",
     "eslint-config-standard": "~6.2.1",
     "eslint-plugin-promise": "~3.4.0",