Support PSR-17 and PSR-18

[Nyholm]

This will drop PHP5 and force us to release a new major version.

[jmalinens]

We are waiting for this!

[mekras]

This issue depends on php-http/discovery#116

[Nyholm]

I’ll put it some work on this during the holidays.

[dbu]

psr-18 support has been added in #43, #52 changes to use PSR-17 instead of the httplug message factory.