Use Digest instead of deprecated Digest::Digest

OpenSSL::Digest::Digest was provided for backwards compatibility, but has been
deprecated in Ruby 2.1.0 giving warnings: ruby/ruby#446

Note that the following specs are failing, but they were failing even before this change:

```
rspec ./spec/aim_spec.rb:222 # AuthorizeNet::AIM::Transaction should be able to run a card present purchase with track 1 data
rspec ./spec/aim_spec.rb:229 # AuthorizeNet::AIM::Transaction should be able to run a card present purchase with LRC codes in the track 1 data
rspec ./spec/aim_spec.rb:236 # AuthorizeNet::AIM::Transaction should be able to run a card present purchase with no LRC or Start/End Sentinels in track 1 data
rspec ./spec/aim_spec.rb:243 # AuthorizeNet::AIM::Transaction should be able to run a card present purchase with track 2 data
rspec ./spec/aim_spec.rb:250 # AuthorizeNet::AIM::Transaction should be able to run a card present purchase with LRC codes in the track 2 data
rspec ./spec/aim_spec.rb:257 # AuthorizeNet::AIM::Transaction should be able to run a card present purchase with no LRC or Start/End Sentinels in track 2 data
rspec ./spec/aim_spec.rb:271 # AuthorizeNet::AIM::Transaction should be able to validate the passed MD5 hash for card present transactions
```

Seeing that they are all related to cart present transactions it may be related to my account.