rework deps+Gemfile+gemspec+Rakefile
add transpec
Convert specs to RSpec 3.1.7 syntax with Transpec

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

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

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

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

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

For more details: https://github.com/yujinakayama/transpec#supported-conversions
remove transpec
remove whitespaces
add rubocop and fix offenses
update rspec config