chore(travis): fix travis by using latest yarn

Author: christopherthielen

.travis.yml

@@ -7,6 +7,10 @@ before_script:
   - sh -e /etc/init.d/xvfb start
   - ./nodeserver.sh > /dev/null &
 
+before_install:
+  - curl -o- -L https://yarnpkg.com/install.sh | bash
+  - export PATH="$HOME/.yarn/bin:$PATH"
+
 matrix:
   allow_failures:
   - env: CORE_BRANCH=