Added yarn install script to use the latest version of yarn.

.travis.yml

@@ -1,5 +1,6 @@
 sudo: required
 language: node_js
+cache: yarn
 os:
 - linux
 - osx
@@ -20,6 +21,8 @@ addons:
 services:
   - xvfb
 before_install:
+- curl -o- -L https://yarnpkg.com/install.sh | bash
+- export PATH="$HOME/.yarn/bin:$PATH"
 - export CHROME_BIN=/usr/bin/google-chrome
 - export DISPLAY=:99.0
 - if [[ "$TRAVIS_OS_NAME" = "linux" ]]; then google-chrome --version; fi