Fix typo in spec_helper.
Convert specs to RSpec 3.0.2 syntax with Transpec

This conversion is done by Transpec 2.3.1 with the following command:
    transpec

* 32 conversions
    from: obj.should
      to: expect(obj).to

* 31 conversions
    from: == expected
      to: eq(expected)

* 3 conversions
    from: obj.should_receive(:message)
      to: expect(obj).to receive(:message)

* 1 conversion
    from: lambda { }.should_not
      to: expect { }.not_to

* 1 conversion
    from: obj.should_not_receive(:message)
      to: expect(obj).not_to receive(:message)

For more details: https://github.com/yujinakayama/transpec#supported-conversions