Add support for Guzzle version 5

[samdjstevens]

Would you be interested in a PR that adds support for Guzzle v5 as well as v6?

I have just experienced a problem trying to use this package in the same project as the official Facebook PHP SDK which requires Guzzle v5 explicitly for now.

I think it would be a small addition to support both versions, and means that the library can be used in more projects that need to remain on Guzzle 5.

[PeteLawrence]

Hi Sam,

Yes please, that'd be useful.

Pete