Gulp build will also copy the system folder (alextselegidis#1207)

gulpfile.js

@@ -36,7 +36,7 @@ gulp.task('package', (done) => {
  fs.copySync('application', 'build/application');
  fs.copySync('assets', 'build/assets');
  fs.copySync('engine', 'build/engine');
- fs.copySync('engine', 'build/system');
+ fs.copySync('system', 'build/system');
 
  fs.ensureDirSync('build/storage/backups');
  fs.copySync('storage/backups/.htaccess', 'build/storage/backups/.htaccess');