chore(travis) use yarn

Author: bertho-zero

.travis.yml

@@ -21,6 +21,9 @@ before_install:
   - curl -o- -L https://yarnpkg.com/install.sh | bash
   - export PATH="$HOME/.yarn/bin:$PATH"
 
+install:
+  - yarn
+
 before_script:
   - export DISPLAY=:99.0
   - sh -e /etc/init.d/xvfb start