[fix] #1192 Set version of Node to ">=6 <9"

package.json

@@ -2,7 +2,7 @@
   "name": "duniter",
   "version": "1.6.13",
   "engines": {
-    "node": ">=6.11.1",
+    "node": ">=6 <9",
     "npm": ">=3.10"
   },
   "engineStrict": true,