Update guzzle dependency

[SneherAdor]

To remove guzzle dependency with latest version:

baileyherbert/envato 3.0.0 requires guzzlehttp/guzzle ^6.5.5 -> found guzzlehttp/guzzle[6.5.5, ..., 6.5.x-dev]

[baileyherbert]

Thanks for the pull request, and apologies that I never got back to you on this. Replacing the guzzle version completely is not ideal, as this would bump the package's minimum PHP version from 5.6 to 7.2.5.

Instead, #23 supports both Guzzle 6 and 7 simultaneously, and is backed by unit tests in two different PHP runtimes. I've merged those changes and will be publishing it as a new minor version shortly.