Reverted part of Loader.php file because of issues with loading configuration

Hi. I reverted part of Loader.php from this commit:
```
https://github.com/nenadalm/Behat/blob/7f84418e43c2ee6e07b12dff0ff00d6d62a3f355/src/Behat/Behat/DependencyInjection/Configuration/Loader.php

```
because of problems with configuration.

We are using behat.yml
```yml
default:
...
    extensions:
        Behat\MinkExtension\Extension:
            base_url: 'http://test.website.com/'
            default_session: goutte
            javascript_session: selenium2
            selenium: ~
            selenium2: ~
            goutte:
                guzzle_parameters:
                    redirect.disable: true
            browser_name: firefox
...
imports:
    - app/config/parameters_behat_test.yml
```


```yml
default:   
    extensions:
        Behat\MinkExtension\Extension:
            base_url: 'http://test-something.website.com/'
```

and without change I sent ```base_url: 'http://test.website.com/'``` is used.