Install yarn when not there

.travis.yml

@@ -192,6 +192,9 @@ before_install:
     if [[ "${TRAVIS_OS_NAME}" == "osx" ]]; then
       export JOBS=$((`sysctl -n hw.ncpu` + 1))
       sudo mdutil -i off /
+    fi
+  - |
+    if [[ ! -f $(which yarn) ]]; then
       npm install -g yarn
     fi
   - export PACKAGE_JSON_VERSION=$(node -e "console.log(require('./package.json').version)")