Travis : Add Yarn version and Node version

Author: Tien Thinh

.travis.yml

@@ -1,13 +1,20 @@
 language: node_js
 
+node_js:
+ - "8"
+
 before_install:
   - nvm install 8.0.0
-  - npm install -g yarn
+  - curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version 1.9.4
+  - export PATH="$HOME/.yarn/bin:$PATH"
+
+install:
   - yarn
 
 cache:
+  yarn: true
   directories:
-    - node_modules
+    - node_modules'
 
 script:
   - yarn run build