Update check-elb-nodes for AWS-SDK v2

[eheydrick]

Pull Request Checklist

Is this in reference to an existing issue?
#240

General

• Update Changelog following the conventions laid out here

• Update README with any necessary configuration snippets

• Binstubs are created if needed

• RuboCop passes

• Existing tests pass

Purpose

Update for AWS-SDK v2. This improves performance and reliability and gets us closer to being able to remove support for v1.

OK state

$ ./check-elb-nodes.rb -n some-elb -r us-west-2
CheckELBNodes OK: InService: 2 (i-87a3d35a, i-a0c81534); OutOfService: 0; Unknown: 0

Warning state

$ ./check-elb-nodes.rb -n some-elb -r us-west-2 -w 1
CheckELBNodes WARNING: InService: 1 (i-87a3d35a); OutOfService: 1 (i-a0c81534); Unknown: 0

Known Compatibility Issues

This is a breaking change because it removes aws_access_key and aws_secret_access_key options per #2.

[majormoses]

LGTM