Use AssumeRoleCredentials for auto-refreshing tokens

[reid-harrison]

https://github.com/aws/aws-sdk-ruby/blob/master/gems/aws-sdk-core/lib/aws-sdk-core/assume_role_credentials.rb

Could be used in place of the existing assume_role call: https://github.com/manheim/awssume/blob/master/lib/awssume/adapter/aws_client.rb#L14