Yarn install from Debian package repository (#5899)

.travis.yml

@@ -27,11 +27,14 @@ addons:
   apt:
     sources:
     - trusty-media
+    - sourceline: deb https://dl.yarnpkg.com/debian/ stable main
+      key_url: https://dl.yarnpkg.com/debian/pubkey.gpg
     packages:
     - ffmpeg
+    - libicu-dev
     - libprotobuf-dev
     - protobuf-compiler
-    - libicu-dev
+    - yarn
 
 rvm:
   - 2.3.4
@@ -42,7 +45,6 @@ services:
 
 install:
   - nvm install
-  - npm install -g yarn
   - bundle install --path=vendor/bundle --without development production --retry=3 --jobs=16
   - yarn install