fix: install newer yarn version on CI

Author: urish

.travis.yml

@@ -3,5 +3,8 @@ sudo: false
 node_js:
   - 8
   - stable
+before_install:
+  - curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version 1.5.1
+  - export PATH=$HOME/.yarn/bin:$PATH
 script:
   - yarn test:ci