Remove lead slash

Addresses FriendsOfPHP/PHP-CS-Fixer#705
Right. Please check it:
```php
<?php

use \C;
use \C\X;

namespace Foo {
    use \A;
    use \A\X;
    
    new X();
}

namespace Bar {
    use \B;
    use \B\X;

    new X();
}
```

Works great on PHP 5.5.15