use Guard::Compat
fix broken specs
add transpec
Convert specs to RSpec 2.99.2 syntax with Transpec

This conversion is done by Transpec 2.3.8 with the following command:
    transpec

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

* 9 conversions
    from: obj.stub(:message)
      to: allow(obj).to receive(:message)

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

For more details: https://github.com/yujinakayama/transpec#supported-conversions
reorganize Gemfile
use Guard::Compat for guard methods
update RSpec to 3.1