require dom, mbstring extensions

without it i get ugly error:

```
$ bin/behat --init
PHP Fatal error:  Class 'DOMDocument' not found in vendor/symfony/config/Symfony/Component/Config/Util/XmlUtils.php on line 52
PHP Fatal error:  Call to undefined function Behat\Behat\DependencyInjection\mb_internal_encoding() in vendor/behat/behat/src/Behat/Behat/DependencyInjection/BehatExtension.php on line 54
```
with it it get nice error from composer:
```
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - The requested PHP extension ext-dom * is missing from your system.
  Problem 2
    - The requested PHP extension ext-mbstring * is missing from your system.
```