add ability to replace existing Dyn records

Add the ability to replace existing records, instead of just throwing Excon::Errors::BadRequest.
Either is accurate, IMO. We're updating/replacing the record at Dyn with the version we're submitting.

TBH, I'm cribbing a bit from the dynect_rest gem, which also uses 'replace':  https://github.com/adamhjk/dynect_rest/blob/master/lib/dynect_rest/resource.rb#L105