Update node engines to match node versions we are testing against (#225)

Update node engines to match node versions we are testing against

Author: Robert Jackson

.travis.yml

@@ -1,6 +1,6 @@
 language: node_js
 node_js:
   - "stable"
-  - "6"
+  - "12"
+  - "10"
   - "8"
-  - "10"

package.json

@@ -15,7 +15,7 @@
     "url": "git+https://github.com/ember-fastboot/fastboot.git"
   },
   "engines": {
-    "node": "^6.14.0 || ^8.10.0 || >=9.10.0"
+    "node": "^8.10.0 || 10.* || >=12"
   },
   "keywords": [
     "ember",