gemspec instead of Gemfile
Bump rspec to 3.
Convert specs to RSpec 3.0.2 syntax with Transpec

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

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

* 5 conversions
    from: obj.should_not
      to: expect(obj).not_to

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