Raise error on 4xx/5xx response

[boardfish]

Feature Request Summary

In the event of an erroneous response, I'd like to be able to raise an exception rather than look for those error conditions manually. Faraday can already do this according to the spec I desire using the Faraday::Response::RaiseError middleware, but in this case I'm using sendgrid-ruby to make requests to SendGrid.

To introduce the change in a non-breaking way, this gem shouldn't raise errors by default.