Change how yarn is installed on travis

Author: existentialism

.travis.yml

@@ -22,7 +22,7 @@ before_install:
  - nvm use $TRAVIS_NODE_VERSION
  - npm set loglevel error
  - npm set progress false
- - 'if [ $PKG_CMD = "yarn" ]; then npm i -g yarn@0.22.0 ; fi'
+ - 'if [ $PKG_CMD = "yarn" ]; then curl -o- -L https://yarnpkg.com/install.sh | bash && PATH=$HOME/.yarn/bin:$PATH ; fi'
 install:
  - $PKG_CMD install
 script: