Infra: Introduce npm/yarn

[EthanLin-TWer]

• [x] npm init
• [x] npm install -g yarn
• [x] commit yarn.lock file
• [x] run npm test in travis script section
• [x] add node_modules to .gitignore file
• [x] cache yarn in travis

Pitfall: use yarn to install dependencies every time after yarn.lock is introduced instead of using npm. Otherwise dependencies can't get resolved correctly by travis calling yarn