Adding Yarn for use instead of npm

See the following issue for details: https://gitlab.com/gitlab-org/gitlab-ce/issues/27613

But essentially we need to be able to lock down sub-dependancies

scripts/install-node

@@ -3,4 +3,6 @@
 set -xeo pipefail
 curl --silent --location https://deb.nodesource.com/setup_7.x | bash -
-apt-get install -y nodejs
\ No newline at end of file
+apt-get install -y nodejs
+
+curl -o- -L https://yarnpkg.com/install.sh | bash