Travis: Install the latest version of yarn, which supports workspaces

.travis.yml

@@ -14,6 +14,10 @@ notifications:
     on_start: never # options: [always|never|change] default: always
 after_success:
   - npm run semantic-release
+before_install:
+  - curl -o- -L https://yarnpkg.com/install.sh | bash
+  - export PATH="$HOME/.yarn/bin:$PATH"
+  - rm -rf node_modules packages/*/node_modules
 branches:
   except:
     - /^v\d+\.\d+\.\d+$/