Add a Guzzle certificate client and add Guzzle 6 as a suggested dependency

[jeskew]

file_get_contents triggers errors and warnings rather than throwing exceptions, which can make recovering from a failure state tricky. This PR adds Guzzle as an optional dependency and provides a client that matches the interface expected by Aws\Sns\MessageValidator. Since Guzzle will also use cURL if it's available, this PR should resolve #23

/cc @kstich

[jeskew]

The test of the optional Guzzle-based client cannot be run on PHP 5.4, so this PR also allows failures on that environment.

[kstich]

This feature is useful for consumers of the package but I don't think we should provide it in a mixed state of compatibility. This probably means taking a major version and updating the minimum PHP version to at least 5.5. We should be able to safely do this, as 5.4 is approaching two years past its EOL, but that should be a conscious decision.

[jeskew]

@kstich I feel the same way. I originally wrote this mostly to serve as an example of how to replace file_get_contents, so perhaps it would work better in a documentation file (alongside the example provided in #23).

[github-actions]

We have noticed this issue has not recieved attention in 3 years. We will close this issue for now. If you think this is in error, please feel free to comment and reopen the issue.

[yenfryherrerafeliz]

Hi @jeskew, thanks for opening this PR. I will close this PR now due to its age, but please if you think this is still needed, please open a new PR over the latest changes in the code base.

Thanks!