ci: update CI config and engines node to run on node 8 and upper

Author: GreatWizard

.travis.yml

@@ -3,7 +3,7 @@ language: node_js
 node_js:
   # we recommend testing addons with the same minimum supported node version as Ember CLI
   # so that your addon works for all apps
-  - "6"
+  - "8"
 
 sudo: false
 dist: trusty

package.json

@@ -47,7 +47,7 @@
     "moment-timezone": "^0.5.0"
   },
   "engines": {
-    "node": "^4.5 || 6.* || >= 7.*"
+    "node": "^8.* || >= 10.*"
   },
   "ember-addon": {
     "configPath": "tests/dummy/config"