Guzzle 6 support

[themondays]

• seegno/uphold-sdk-php 5.0.1 requires guzzlehttp/guzzle ~5.1 -> satisfiable by guzzlehttp/guzzle[5.1.0, 5.2.0, 5.3.0, 5.3.1, 5.3.2, 5.3.x-dev] but these conflict with your requirements or minimum-stability.

[nunorafaelrocha]

Hi @themondays,

Thanks for the issue. Over the next weeks, we may not be able to update this project to support Guzzle 6, so any PRs are welcome!