Set the minimum node engine to 8.9.4

PI-366

Author: erodewald

package.json

@@ -17,7 +17,7 @@
     "lint": "xo"
   },
   "engines": {
-    "node": ">=6.12.3 <8.0.0 || >=8.9.4 <10.0.0 || >=10.4.1"
+    "node": ">=8.9.4"
   },
   "license": "Apache-2.0",
   "repository": "github:SmartThingsCommunity/smartapp-sdk-nodejs",