Replace PSR-2 with PSR-12

As of 2019-08-10 PSR-2 has been marked as deprecated. [PSR-12](https://www.php-fig.org/psr/psr-12/) is now recommended as an alternative.
- Update [.styleci.yml](https://github.com/botman/driver-telegram/blob/master/.styleci.yml)
- Update [CONTRIBUTING.md](https://github.com/botman/driver-telegram/blob/master/CONTRIBUTING.md)
- Anything else?

Start with .styleci.yml to identify the current PSR-2 configuration, then read CONTRIBUTING.md for coding-standard references. Compare both with the linked PSR-12 guidance and update the stated standard and configuration; done means no remaining PSR-2 references in those files or related configuration.

Written by the indexing model from the issue text.