ci(Travis): install latest yarn from repo

.travis.yml

@@ -1,13 +1,15 @@
 language: node_js
 cache:
   yarn: true
-  directories:
-    - node_modules
 notifications:
   email: false
 node_js: 'node'
+sudo: required
 before_install:
-- npm install -g yarn@latest
+- 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
 - yarn global add greenkeeper-lockfile@latest
 before_script: greenkeeper-lockfile-update
 after_script: greenkeeper-lockfile-upload