Start webpack-dev-server directly

Starting with `npm run` creates an intermediary `npm` process under
Foreman, which prevents Foreman from shutting down webpack-dev-server.

Procfile.example

@@ -14,4 +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 /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 node_modules/.bin/webpack-dev-server --config config/webpack.config.js