Relieve `multipart-post` version constraint for Ruby 2.0 compatibility

`multipart-post` versions prior to 1.2.0 are not compatible with Ruby 2.0; sending a request results in I/O errors (see https://github.com/nicksieger/multipart-post/pull/25).

I'm not sure if there was a specific reason why the version was previously pinned at 1.1.5. If removing that constraint is a problem, let me know, but the gem seems to be working fine on Ruby 2.0 now that it is gone.