Remove theseer/autoload from composer.json

It has not yet been submitted to packagist and therefore causes the build to fail because it cannot resolve dependencies. Initial support has been added for installation via Composer (https://github.com/theseer/Autoload/commit/94baad8ab08687615cb7b3ded42e1bcfbb209e03) but until it has been submitted to packagist, we cannot require this dependency.