chore: add Node.js 8 and 10, remove Node.js 4 (#829)

Author: DanielRuf

.travis.yml

@@ -1,6 +1,7 @@
 sudo: false
 language: node_js
 node_js:
+  - 8
   - 6
   - 4
   - "stable"

appveyor.yml

@@ -1,7 +1,9 @@
 environment:
   matrix:
-    - nodejs_version: '6' # stable
-    - nodejs_version: '4' # LTS
+    - nodejs_version: '10' # stable
+    - nodejs_version: '8' # LTS
+    - nodejs_version: '6'
+    - nodejs_version: '4'
 install:
   - ps: Install-Product node $env:nodejs_version
   - npm cache clear