Force travis to use most recent yarn version

See travis-ci/travis-ci#7566

Author: Markus Oberlehner

.travis.yml

@@ -1,6 +1,9 @@
 language: node_js
 node_js:
   - "node"
+before_install:
+  - curl -o- -L https://yarnpkg.com/install.sh | bash
+  - export PATH="$HOME/.yarn/bin:$PATH"
 script:
   - yarn run lint
   - yarn test -- --maxWorkers=4