Added webpack to Procfile

Adds webpack dev server command to Procfile

Procfile.example

@@ -14,3 +14,4 @@ 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 /usr/bin/env GITALY_SOCKET_PATH=/home/git/gitaly.socket GITALY_PROMETHEUS_LISTEN_ADDR=:9101 gitaly/bin/gitaly
+webpack: exec support/exec-cd gitlab npm run dev-server