Bunch of updates to make the gem work for Versa.

(Sorry to bunch all of these commits into one PR.)

My company, [Versa](https://versahq.com) (formerly ElectNext) is a pipedrive customer and we use (our fork of) this gem to integrate our contact form with pipedrive.

In order to get the gem to work for our use case we needed to make some updates. Feel free to cherry pick anything here, but I think they will all be helpful. Especially [this one](https://github.com/ElectNext/pipedriver/commit/6fff777f499e4eeab2fa5f738ef5348ac229365e) and [this one](https://github.com/ElectNext/pipedriver/commit/a6aaa3f4ae8beb8652db638d5046cb89a23d971e) as they seem to be crucial. Also the [readme update](https://github.com/ElectNext/pipedriver/commit/11342382d596f63574d27485aced13ebb5e968b1) might help anyone trying to get this running in Rails.