Use latest yarn (#57)

.travis.yml

@@ -14,6 +14,8 @@ language: node_js
 node_js: "8"
 
 before_install:
+  - curl -o- -L https://yarnpkg.com/install.sh | bash
+  - export PATH="$HOME/.yarn/bin:$PATH"
   - ./install_deps.sh
   - . get_repo.sh
   - . check_tags.sh

install_deps.sh

@@ -2,7 +2,6 @@
 
 if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then
   brew update
-  brew install yarn --without-node
   brew install jq zip
 else
   sudo apt-get update