Update Guzzle dependency to v5.x

[knasher]

Guzzle 3 (https://packagist.org/packages/guzzle/guzzle & https://github.com/guzzle/guzzle3) is now deprecated in favour of Guzzle 5 (https://packagist.org/packages/guzzlehttp/guzzle & https://github.com/guzzle/guzzle).

Update Guzzle dependency and code/tests accordingly, making sure service descriptions functionality is retained.

[eddiejaoude]

👍

[eddiejaoude]

Any ETA on this @knasher ? It is blocking other libraries who need v5.