Upgrade yarn to latest version on Travis

Author: kunalkapadia

.travis.yml

@@ -13,3 +13,6 @@ script:
   - yarn test:check-coverage
 after_script:
   - yarn report-coverage
+before_install:
+  - curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version 1.5.1
+  - export PATH=$HOME/.yarn/bin:$PATH