Fix coap version

Author: lizhizhou

package.json

@@ -51,7 +51,7 @@
     "tingyun": "*",
     "mathjs": "*",
     "microtime": "^2.1.2",
-    "coap": "^0.21.0",
+    "coap": "0.17.0",
     "helmet": "^3.6.0",
     "numeric": "*"
   },