upgrade deps

package.json

@@ -3,7 +3,7 @@
   "version": "5.2.0",
   "description": "MySQL connector for loopback-datasource-juggler",
   "engines": {
-    "node": ">=4"
+    "node": ">=6"
   },
   "main": "index.js",
   "scripts": {
@@ -13,7 +13,7 @@
     "posttest": "npm run lint"
   },
   "dependencies": {
-    "async": "^0.9.0",
+    "async": "^2.6.1",
     "debug": "^3.1.0",
     "lodash": "^4.17.4",
     "loopback-connector": "^4.0.0",