chore: test only against Node.js 8 and 10

.appveyor.yml

@@ -4,10 +4,8 @@ build: off
 platform: x64
 environment:
   matrix:
-    - nodejs_version: 7
-    - nodejs_version: 6
-    - nodejs_version: 5
-    - nodejs_version: 4
+    - nodejs_version: 10
+    - nodejs_version: 8
 install:
   - ps: Install-Product node $env:nodejs_version x64
   - set CI=true

.travis.yml

@@ -1,8 +1,6 @@
 language: node_js
 node_js:
-  - 7
-  - 6
-  - 5
-  - 4
+  - 10
+  - 8
 before_install:
-  - npm i -g npm
+  - npm i -g npm@latest