Define the upcoming version of July

lib/shopify_api/defined_versions.rb

@@ -4,6 +4,7 @@ module ShopifyAPI
     def define_known_versions
       define_version(ShopifyAPI::ApiVersion::Unstable.new)
       define_version(ShopifyAPI::ApiVersion::Release.new('2019-04'))
+      define_version(ShopifyAPI::ApiVersion::Release.new('2019-07'))
     end
   end
 end