fix(package): update superagent to version 3.8.1

Author: greenkeeper[bot]

package.json

@@ -11,7 +11,7 @@
     "prepublish": "npm build"
   },
   "dependencies": {
-    "superagent": "3.8.0",
+    "superagent": "3.8.1",
     "uglify-js": "^3.1.3"
   },
   "devDependencies": {