Convert specs to RSpec 3.0.2 syntax with Transpec

This conversion is done by Transpec 2.3.3 with the following command:
    transpec -f

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

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

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

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

* 3 conversions
    from: its([:key]) { }
      to: describe '[:key]' do subject { super()[:key] }; it { } end

* 2 conversions
    from: it { should_not ... }
      to: it { is_expected.not_to ... }

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

For more details: https://github.com/yujinakayama/transpec#supported-conversions
Remove deprecated RSpec configuration

RSpec::Core::Configuration#treat_symbols_as_metadata_keys_with_true_values=
is deprecated, it is now set to true as default and setting it to false
has no effect.
Merge pull request #14 from meganemura/introduce-rspec3

Introduce RSpec 3.0.2 syntax
[WIP]