Use permutation rather than combination

While it requires twice as many elements to check were not working with
that many and using permutation avoids needed to ensure the list is
sorted.

app/models/policies.rb

@@ -56,8 +56,8 @@ def self.with_attribute(attr)
   ]
 
   def self.prohibited_policy_combination?(policies)
-    policies.combination(2).any? do |policy1, policy2|
-      INVALID_POLICY_COMBINATIONS.include?([policy1, policy2].sort_by(&:to_s))
+    policies.permutation(2).any? do |policy1, policy2|
+      INVALID_POLICY_COMBINATIONS.include?([policy1, policy2])
     end
   end
 end

spec/models/policies_spec.rb

@@ -58,12 +58,4 @@
       end
     end
   end
-
-  describe "::INVALID_POLICY_COMBINATIONS" do
-    it "contains subarrays that are sorted alphabetically" do
-      Policies::INVALID_POLICY_COMBINATIONS.each do |subarray|
-        expect(subarray).to eq(subarray.sort_by(&:to_s))
-      end
-    end
-  end
 end