use yarn to run webpack-dev-server in Procfile

Procfile.example

@@ -15,4 +15,4 @@ rails-background-jobs: exec /usr/bin/env SIDEKIQ_LOG_ARGUMENTS=1 RAILS_ENV=devel
 #sshd: exec /usr/sbin/sshd -e -D -f /home/git/openssh/sshd_config
 #nginx: exec nginx -p /home/git/nginx -c conf/nginx.conf
 gitaly: exec gitaly/bin/gitaly /home/git/gitaly/config.toml
-webpack: exec /usr/bin/env NODE_ENV=development DEV_SERVER_PORT=3808 support/exec-cd gitlab npm run dev-server
+webpack: exec /usr/bin/env NODE_ENV=development DEV_SERVER_PORT=3808 support/exec-cd gitlab yarn dev-server