upgrade guzzle

[J0WI]

Forked from #75:

We currently have problems with dependencies:

Package guzzle/guzzle is abandoned, you should avoid using it. Use guzzlehttp/guzzle instead.
Package misd/guzzle-bundle is abandoned, you should avoid using it. No replacement was suggested.

[freaktechnik]

We might end up having to do a bigger dependency upgrade, since most guzzle bundles I see require a newer version of Symfony, which we currently have hard locked.

[MichaelKohler]

@freaktechnik would you be interested in doing this? I will create another PR within the next few days and that would be a great opportunity to release a new version