Bugfix/encode decode activeresource versions

@csaunders @maartenvg 

Setting `include_root_in_json` to true turned out to be problematic for ActiveResource version 3.2, so I yanked 3.1.1. This reinstates the encode/decode/as_json overrides pretty much to the way they were in 3.0.3 with the exception that the decode override is only necessary for ActiveResource 3.0. The other change is that the encode/as_json overrides are scoped to ShopifyAPI::Base rather than ActiveRecord::Base so we don't tromp expectations from other gems.

Original PR is #67 for reference.