Drop Node 6 and 11 support.

.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"
 
 addons:
   chrome: stable

package.json

@@ -15,7 +15,7 @@
     "test:node:debug": "mocha debug node-tests/*.js"
   },
   "engines": {
-    "node": "6.* || 8.* || >= 10.*"
+    "node": "8.* || 10.* || >= 12.*"
   },
   "devDependencies": {
     "@ember/optional-features": "^0.7.0",