chore: test Node.js 6, 8 and 10 (#1160)

BREAKING CHANGE: drop Node 6 from testing matrix, such that we'll gradually start drifting away from supporting Node 4.

.travis.yml

@@ -7,6 +7,6 @@ os:
 after_success: npm run coverage
 
 node_js:
-  - "4"
   - "6"
-  - "node"
+  - "8"
+  - "10"

appveyor.yml

@@ -3,16 +3,16 @@
 # Test against these versions of Node.js.
 environment:
   matrix:
-    - nodejs_version: "4"
     - nodejs_version: "6"
-    - nodejs_version: "7"
+    - nodejs_version: "8"
+    - nodejs_version: "10"
 
 # Install scripts. (runs after repo cloning)
 install:
   # Get the latest stable version of Node 0.STABLE.latest
   - ps: Install-Product node $env:nodejs_version
   # Typical npm stuff.
-  - npm i npm@5
+  - npm i -g npm@latest
   - npm install
 
 # Post-install test scripts.