fix hanging when testing signal

fixed in arthurdandrea/rbuv
Update rspec syntax and lock rspec dependency to version 2.x
Convert specs to RSpec 2.99.1 syntax with Transpec

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

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

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

* 2 conversions
    from: it { should ... }
      to: it { is_expected.to ... }

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

* 1 conversion
    from: pending
      to: skip

For more details: https://github.com/yujinakayama/transpec#supported-conversions
update rspec to >= 2.99 and rspec --init
update .travis.yml
Only skip spec Rbuv::Tcp#bind on linux, and show error if it passes