Allow for newer Guzzle versions

I had some major issues integrating your library today because you are using a very out of date Guzzle.  The only version of AWS's library that worked with this year old Guzzle version was out of date and missing some crucial methods.  I didn't see any [forwards compatibility issues](https://github.com/guzzle/guzzle/releases) when allowing for a year range of Guzzle libraries.