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 IO errors.

See https://github.com/nicksieger/multipart-post/pull/25.
Merge pull request #13 from FoodLogiQ/multipart-post-ruby_2.0

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