Updated funnelback-php to use Guzzle 6.

[matason]

Hi Kim,

A bit of preliminary work ahead of adding support for push.

• Bumped Guzzle to 6.
• Added support for passing in a Guzzle Handler into the Client - http://docs.guzzlephp.org/en/latest/handlers-and-middleware.html (used in ClientTest.php)
• Test changed to Funnelback\Client instead of GuzzleHttp\Client.

[kimpepper]

Thanks for your work.

There are quite a lot of changes in here. Would be easier to review if you broke it up into related chunks and submitted separate PRs.

[matason]

Hi Kim,

The update to Guzzle 6 is fairly atomic but I'll see what I can do.

Thanks,

Chris