AWS::SQS::Queue - use OpenSSL for Digest needs

[fallwith]

As with #529 (ee75b78), I would like for AWS::SQS::Queue to switch to using OpenSSL::Digest instead of Digest. Without this change I am running into Digest::Base cannot be directly inherited in Ruby when running multiple MRI threads.

[fallwith]

Can someone please take a look at this one?

It looks like every other instance of Digest::MD5 was already converted to OpenSSL::Digest::MD5, and this PR seeks to make that same change to the last remaining instance of Digest::MD5 in the AWS::SQS::Queue class.

Currently we're using multiple threads to listen to SQS queues (one unique queue per thread) and occasionally everything blows up when it hits the Digest::MD5 line. I researched it a bit and found that Digest::MD5 is not thread-safe and that using OpenSSL::Digest::MD5 solves the problem. Then I discovered that the aws-sdk-ruby project had already made the change to OpenSSL for all but one instance of Digest::MD5, so this PR was submitted to get that last instance changed as well.

[trevorrowe]

Thanks for the contribution. This should go out with the next release.