[RFC] moved the vendor to phpoffice/phpexcel

In order to use the official PHPOffice\PHPExcel.
I also created an issue PHPOffice/PHPExcel#169 hoping they finally clean the tags.

- [ ] Gather feedback

1. pro: it uses official repo, almost ready for tagging clean-up.
2. cons: not so easy to install, old versions and new-one are (for now) not supported.

In one week, this PR will be accepted, please insert your comment below.