chore(travis): manually install yarn

Currently, the yarn version on Travis is outdated:
travis-ci/packer-templates#478

Author: Narretz

.travis.yml

@@ -36,6 +36,10 @@ addons:
     packages:
       - g++-4.8
 
+before_install:
+  - curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version 0.27.5
+  - export PATH="$HOME/.yarn/bin:$PATH"
+
 before_script:
   - du -sh ./node_modules ./bower_components/ || true
   - "./scripts/travis/before_build.sh"