Using spot instances required as [upgrade of AWS-SDK](https://github.com/mrdavidlaing/bosh/commit/240369853e5c9554d2c5c81f6a50f45811cd7af8).  Unfortunately, this caused [these (unrelated) tests to start failing](https://travis-ci.org/cloudfoundry/bosh/jobs/17593126#L1133):
```
  1) Bosh::Aws::RDS subnet_group_exists? should return false if the db subnet group does not exist
     Failure/Error: fake_aws_rds_client.should_receive(:describe_db_subnet_groups).
       AWS::RDS::Client does not implement:
         describe_db_subnet_groups
     # ./spec/unit/rds_spec.rb:17:in `block (3 levels) in <top (required)>'
```

As [discussed in the comment above](https://github.com/cloudfoundry/bosh/pull/509#issuecomment-33284205), I think  `let(:fake_aws_rds_client) { instance_double('AWS::RDS::Client') }` is getting confused by the [dynamic method generation used by the AWS-SDK](https://github.com/aws/aws-sdk-ruby/blob/master/lib/aws/rds/client.rb#L31).  

Switching to `double` seemed the smallest change that fixed that confusion, allowing all tests to start passing.

Can you think of a better way?