Convert specs to RSpec 2.99.2 syntax with Transpec

Also remove rspec-its

This conversion is done by Transpec 2.3.7 with the following command:
    transpec

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

* 5 conversions
    from: its(:attr) { }
      to: describe '#attr' do subject { super().attr }; it { } end

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

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