Remove lead slash

Addresses FriendsOfPHP/PHP-CS-Fixer#705
Just to make it clear... Because I might have misunderstood. Shouldn't Tokens::getNamespaceUseIndexes return _only_ the T_USE tokens within the scope of namespaces? Why does it return also those outside?

https://github.com/FriendsOfPHP/PHP-CS-Fixer/blob/master/Symfony/CS/Tokenizer/Tokens.php#L499-L505