Guzzle is locked to deprecated version

Am using Guzzle 7, as that is stable, and that is required by other packages.

But Mailjet is using Guzzle v6.   

Composer does not seem to have a way for two different packages to be using different versions of Guzzle.

Have solved it for now by moving over to Sendgrid instead of Mailjet, but think it should be fixed.
