Pin qs to version 6.15.1

Author: depfu[bot]

package.json

@@ -63,7 +63,7 @@
     "is-stream": "^2.0.0",
     "lodash": "^4.0.0",
     "messy": "^7.0.0",
-    "qs": "^6.5.1",
+    "qs": "6.15.1",
     "unexpected-messy": "^11.1.0"
   },
   "nyc": {