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.
Remove spec_helper

This isn't necessary anymore because Savon does not have a global configuration
manager in version 2. Instead, when you instantiate a client, you have an option
to pass in some options there, and you can also pass in options (that will take
precedence over the previous ones) when you call the #call method on a Savon::Client

Neat, and with way less global state than before. Either way, this spec helper isn't
needed anymore.
Update rspec tests

Nothing crazy, just changed how we create the response
Cleanup some whitespace
Add tests for the #body and #to_xml methods

I had previously only been using the #to_xml and #parts methods, so I hadn't
run this this before. The issue was that Nori does not know how to handle an
instance of Mail::Body rather than a string. It seems that the correct thing
to do is to just return a string.

Also brought in some same defaults from Savon::GlobalOptions for this test.
Before it wouldn't have been possible to actually call the #body method in
the tests because we didn't specify how to take the response body tags and
turn them into snakecase symbols
Update tests to pass on 1.8.7

Unsure whether this will pass on ree, but it will pass on all
supported versions of 1.8.7 (p371 mri, jruby18-mode and rubinius).