feat: cache dependencies in ci building

.travis.yml

@@ -2,6 +2,10 @@ language: node_js
 node_js:
   - "8"
   - "6"
+cache:
+  yarn: true
+  directories:
+    - node_modules
 install:
   - yarn install
   - yarn run build

appveyor.yml

@@ -4,6 +4,10 @@ environment:
     - nodejs_version: "6"
     - nodejs_version: "8"
 
+cache:
+  - "%LOCALAPPDATA%\\Yarn"
+  - node_modules
+
 # Install scripts. (runs after repo cloning)
 install:
   # Get the latest stable version of Node.js or io.js