Some improve

- Fix `z-index` on `bootstrap-select.show-menu-arrow` (last commit in master: https://github.com/silviomoreto/bootstrap-select/blob/5448892728eb9607b2b1f0fb62305420544d6914/dist/css/bootstrap-select.css#L178)
- Remove outdated `.muted` class: http://getbootstrap.com/migration/#classes
- `clean` task already called in `build-css` and `build-js` tasks.
- Remove overlapping color.
  - https://github.com/silviomoreto/bootstrap-select/blob/5448892728eb9607b2b1f0fb62305420544d6914/dist/css/bootstrap-select.css#L116-117
  - https://github.com/silviomoreto/bootstrap-select/blob/5448892728eb9607b2b1f0fb62305420544d6914/dist/css/bootstrap-select.css#L186-187
  - https://github.com/silviomoreto/bootstrap-select/blob/5448892728eb9607b2b1f0fb62305420544d6914/dist/css/bootstrap-select.css#L209-210