Add aws-sdk gem to artifact_file load_current_resource

The artifact_file resource requires the aws-sdk gem, the same as the artifact_deploy resource. Fixes the following:

```
LoadError
---------
cannot load such file -- aws-sdk

Cookbook Trace:
---------------
c:/chef/cache/cookbooks/artifact/libraries/chef_artifact.rb:92:in `retrieve_from_s3'
c:/chef/cache/cookbooks/artifact/providers/file.rb:48:in `block in class_from_file'
```

Note: I also bumped the aws-sdk gem version to the current latest (1.29.0).