upgrade travis yarn to 1.5.1

.travis.yml

 language: node_js
 node_js: node
 cache: yarn
+before_install:
+  - curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version 1.5.1
+  - export PATH="$HOME/.yarn/bin:$PATH"
 install: yarn 
 script: yarn lint