chore(package): update async to version 2.6.0 (#2548)

package.json

@@ -55,7 +55,7 @@
   },
   "devDependencies": {
     "angular-mocks": "~1.6.6",
-    "async": "~2.5.0",
+    "async": "~2.6.0",
     "babel": "~6.23.0",
     "babel-core": "~6.26.0",
     "babel-loader": "~7.1.2",