Clean up Gemfile, use https for rubygems

It's unlikely that anyone will ever be MITM'd and receive a bad gem, but
it's a best practice to use ssl for installing gems
Updated README with Savon2 notice
Refactor savon-multipart for Savon2

Needs tests but the interface between savon-multipart and savon just became a
whole lot smaller.
Clean up spec_helper
Update tests

Nothing crazy, just changed how we create the response