Use pre 3.0 compatible stubbing

`receive_messages` is not available pre 3.0. This will let the
test suite run with an earlier version of rspec.

Author: thomas-holmes

spec/rspec/active_model/mocks/mock_model_spec.rb

@@ -229,12 +229,12 @@
           expect(model.column_a).to eq("a")
         end
         it "does not stub column accessor if already stubbed after declaration (with string)" do
-          allow(@model).to receive_messages("column_a" => "a")
+          allow(@model).to receive(:column_a) { "a" }
           @model.respond_to?("column_a")
           expect(@model.column_a).to eq("a")
         end
         it "does not stub column accessor if already stubbed after declaration (with symbol)" do
-          allow(@model).to receive_messages(:column_a => "a")
+          allow(@model).to receive(:column_a) { "a" }
           @model.respond_to?("column_a")
           expect(@model.column_a).to eq("a")
         end