Readme suggests abandoned package php-http/guzzle6-adapter

The readme suggests using `php-http/guzzle6-adapter`, but following that advice leads to a warning
```
Package php-http/guzzle6-adapter is abandoned, you should avoid using it. Use guzzlehttp/guzzle or php-http/guzzle7-adapter instead.
```
Probably, we should update that (the package suggested by the warning seems to work just fine)