remove guzzlehttp/guzzle from require in composer.json

[tacman]

Since #1032 was merged, shouldn't we be able to move guzzle to the suggests key, rather than in require?

I'm using Symfony's http client, so there's no reason to carry guzzle.

I can make a PR -- anything in particular I should watch out for?

[tacman]

oops -- I see, it was merged into 3.0. Is there a timeframe for a 3.0 release?