Support Savon version 2 requests

I wanted to use Savon version 2 and despaired that savon-multipart did not support it, so I've updated this gem and cleaned up as much of the low hanging fruit as I could.

* removed dead code
* changed Gemfile to use https://rubygems
* improved the interface between Savon and Savon-Multipart

The corresponding pull request for Savon is [here](https://github.com/savonrb/savon/pull/434)