chore: always install the latest version of yarn

Author: SimenB

.travis.yml

@@ -5,7 +5,7 @@ node_js:
 - 6
 - 4
 before_install:
-- curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version 1.3.2
+- curl -o- -L https://yarnpkg.com/install.sh
 - export PATH="$HOME/.yarn/bin:$PATH"
 - yarn global add greenkeeper-lockfile@1
 install: yarn --ignore-engines