Add belongs_to optional true to setup_association

lib/impressionist/setup_association.rb

@@ -12,8 +12,12 @@ module Impressionist
     end
 
     def define_belongs_to
+      if ::Rails::VERSION::MAJOR.to_i >= 5
+        receiver.belongs_to(:impressionable, :polymorphic => true, :optional => true)	         
+      else
         receiver.belongs_to(:impressionable, :polymorphic => true)
       end
+    end
 
     def set
       define_belongs_to

tests/spec/minitest_helper.rb

@@ -1,4 +1,7 @@
 $:.unshift(File.dirname __FILE__)
 
+require 'rails/all'
+require 'rails/test_help'
 require "minitest/autorun"
 require "minitest/pride"
+require 'minitest/rails'