Fix lib/rb/spec/union_spec.rb so that CI succeeds

Failures:

  1) Union Thrift::Union should raise for wrong set field when hash initialized and type checking is off

     Failure/Error: expect(example).to raise_error(RuntimeError, "set_field is not valid for this union!")

       You must pass a block rather than an argument to `expect` to use the provided block expectation matcher (raise RuntimeError with "set_field is not valid for this union!").

     # ./spec/union_spec.rb:55:in `block (3 levels) in <top (required)>'

     it "should raise for wrong set field when hash initialized and type checking is off" do
       Thrift.type_checking = false
       union = SpecNamespace::My_union.new({incorrect_field: :incorrect})
-      example = lambda { Thrift::Serializer.new.serialize(union) }
-      expect(example).to raise_error(RuntimeError, "set_field is not valid for this union!")
+      expect { Thrift::Serializer.new.serialize(union) }.to raise_error(RuntimeError, "set_field is not valid for this union!")
     end

    it "should raise for wrong set field" do
      union = SpecNamespace::My_union.new
      union.integer32 = 25
      expect { union.some_characters }.to raise_error(RuntimeError, "some_characters is not union's set field.")
    end