Add required platform attribute
Add required platform attribute

The `platform` attribute is required by the [RubyGems gemspec specification][gemspec-spec].
This change sets it to the local operating system and architecture.

[gemspec-spec]: http://guides.rubygems.org/specification-reference/#platform=

gregory/bigcommerce-api-ruby,mattolson/bigcommerce-api-ruby,bigcommerce/bigcommerce-api-ruby,hasclass/bigcommerce-api-ruby,garrinmf/bigcommerce-api-ruby,pedelman/bigcommerce-api-ruby,sgerrand/bigcommerce-api-ruby,emeraldMaster/bigcommerce-api-ruby