def capturing_output
      output = StringIO.new
      $stdout = output
      yield
      output.string
    ensure
      $stdout = STDOUT
    end

then…

    it "exits immediately from --version" do
      output = capturing_output do
        lambda {
          Erector.new(["--version"])
        }.should raise_error(SystemExit)
      end
      output.should == Erector::VERSION + "n"
    end