Migrate to /guzzle7-adapter

[JulioJu]

Hi,

Currently, https://github.com/php-http/guzzle6-adapter is used. But now there is https://github.com/php-http/guzzle7-adapter .

If I understand their doc, guzzle7-adaptater remove some overhead.

What do you think about migration ?

[cmfcmf]

Hey @JulioJu, good point. Note though, that this repo only uses php-http/guzzle6-adapter as a dev dependency for testing, so users of this library should be able to upgrade without me updating the dev dependency.