Update supported Node.js versions

.travis.yml

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

appveyor.yml

@@ -12,10 +12,12 @@ environment:
       PLATFORM: x64
     - NODEJS_VERSION: "10"
       PLATFORM: x64
+    - NODEJS_VERSION: "12"
+      PLATFORM: x64
 
 install:
   - ps: Install-Product node $env:NODEJS_VERSION $env:PLATFORM
-  - npm install
+  - if "%NODEJS_VERSION%" lss "8" (npm install) else (npm ci) # switch to `npm ci` when Node.js 6.x is dropped
 
 test_script:
   - node --version
@@ -31,4 +33,5 @@ matrix:
   fast_finish: true
 
 cache:
-  - 'node_modules -> appveyor.yml,package.json,package-lock.json'
+  - '%APPDATA%\npm-cache\ -> .appveyor.yml,package.json,package-lock.json'
+  - 'node_modules -> appveyor.yml,package.json,package-lock.json' # drop this when Node.js 6.x is removed