Use modern Yarn version on Travis CI

Travis CI uses a quite old version of Yarn by default. This adds Yarn's
recommended incantation for using the latest stable version.

.travis.yml

@@ -13,6 +13,9 @@ node_js:
     - node # Latest stable version of nodejs.
 addons:
     chrome: stable
+before_install:
+    - curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version 1.13.0
+    - export PATH=$HOME/.yarn/bin:$PATH
 install:
     - ./scripts/travis/install-deps.sh
 matrix: