perf: increase minimum required node version to 8.x

.travis.yml

@@ -1,7 +1,7 @@
 language: node_js
 node_js:
   - "8"
-  - "6"
+  - "9"
 cache:
   yarn: true
   directories:

appveyor.yml

@@ -1,8 +1,8 @@
 # Test against the latest version of this Node.js version
 environment:
   matrix:
-    - nodejs_version: "6"
     - nodejs_version: "8"
+    - nodejs_version: "9"
 
 cache:
   - "%LOCALAPPDATA%\\Yarn"

package.json

@@ -60,8 +60,8 @@
     "release-next": "npm run build && node ./build/release-next && npm publish --tag next"
   },
   "engines": {
-    "node": ">=6.11",
-    "npm": ">=3.10.0"
+    "node": ">=8.0.0",
+    "npm": ">=5.0.0"
   },
   "dependencies": {
     "@nuxtjs/youch": "^3.1.0",