Update composer.json to fit packagist conventions

Adding this package to a new packagist instance results in the error: 

The package name DrSlump/Protobuf-PHP is invalid, it should not contain uppercase characters. We suggest using dr-slump/protobuf-php instead.

I'm guessing this rule was added after this package was already on the central packagist server, but it prevents in house packagists from adding this package now.