require PSRs instead of concrete packages

[garak]

This library should not require directly monolog and guzzle. Instead, it should require psr/log and psr/http-client

Of course, monolog and guzzle could be stille be required in required-dev (since you need concrete packages to run tests) and maybe suggested. But relying on interfaces it's way better.

On a side note, psr/http-client would require to raise minimum version of php to 7.0, that is also good (being php 5.6 out of active support since January 2017)

[akalongman]

@garak we are working on that, you can look at branch https://github.com/php-telegram-bot/core/tree/feature/refactor-app-design

Also, if you have other ideas for package new design, please share with us