Update supported Node.js versions.

* drop Node.js 6.x
* add Node.js 12.x
* tweak CI configs

.travis.yml

@@ -1,15 +1,11 @@
-sudo: false
-
 language: node_js
 
 node_js:
-  - "6"
   - "8"
   - "10"
+  - "12"
 
 matrix:
   fast_finish: true
 
-cache:
-  directories:
-    - node_modules
+cache: npm

appveyor.yml

@@ -4,18 +4,18 @@ version: "{build}"
 
 environment:
   matrix:
-    - NODEJS_VERSION: "6"
+    - NODEJS_VERSION: "8"
       PLATFORM: x86
-    - NODEJS_VERSION: "6"
-      PLATFORM: x64
     - NODEJS_VERSION: "8"
       PLATFORM: x64
     - NODEJS_VERSION: "10"
       PLATFORM: x64
+    - NODEJS_VERSION: "12"
+      PLATFORM: x64
 
 install:
   - ps: Install-Product node $env:NODEJS_VERSION $env:PLATFORM
-  - npm install
+  - npm ci
 
 test_script:
   - node --version
@@ -31,4 +31,4 @@ matrix:
   fast_finish: true
 
 cache:
-  - 'node_modules -> appveyor.yml,package.json,package-lock.json'
+  - '%APPDATA%\npm-cache\ -> appveyor.yml,package.json,package-lock.json'

package.json

@@ -9,7 +9,7 @@
   "repository": "gruntjs/grunt-contrib-internal",
   "license": "MIT",
   "engines": {
-    "node": ">=6"
+    "node": ">=8"
   },
   "scripts": {
     "test": "grunt test"