Roadmap for new version

[Progi1984]

• Migration to Github Actions from Travis CI
• Support for recents versions of PHP and remove support for PHP < 7.1
• Add PHPStan
• Add PHPCSFixer
• Use Coveralls for replacing Scrutinizer
• Remove all deprecated
• Migration documentation to Markdown (@mkdocs)
• Migrate from PHPExcel to PhpSpreadsheet
• Define new exceptions for each error
• Add declare strict_types for each file
• Upgrade major version of PHPOffice\Common

Inspired from PHPOffice/PHPPresentation#634

[PowerKiKi]

I marked PHPStan as added, following #2405