ci: update Yarn with the recommended official commands

Author: Daniel Ruf

.travis.yml

@@ -9,11 +9,14 @@ cache:
     - node_modules
 
 before_install:
+  - sudo apt-key adv --fetch-keys http://dl.yarnpkg.com/debian/pubkey.gpg
+  - echo "deb http://dl.yarnpkg.com/debian/ stable main" | sudo tee /etc/apt/sources.list.d/yarn.list
+  - sudo apt-get update -qq
+  - sudo apt-get install -y -qq yarn
   - chmod +x start-server.sh
 
 before_script:
-  - npm install -g yarn
-  - yarn install
+  - yarn
   - './start-server.sh'
 
 script: