Upgrade RSpec & Convert specs to RSpec 3.1.7 syntax with Transpec

This conversion is done by Transpec 2.3.7 with the following command:
    transpec -f

```
* 171 conversions
    from: obj.should
      to: expect(obj).to

* 109 conversions
    from: == expected
      to: eq(expected)

* 43 conversions
    from: obj.should_receive(:message)
      to: expect(obj).to receive(:message)

* 15 conversions
    from: it { should ... }
      to: it { is_expected.to ... }

* 14 conversions
    from: obj.stub(:message)
      to: allow(obj).to receive(:message)

* 4 conversions
    from: lambda { }.should
      to: expect { }.to

* 2 conversions
    from: obj.should_not
      to: expect(obj).not_to

* 2 conversions
    from: obj.should_not_receive(:message)
      to: expect(obj).not_to receive(:message)

* 2 conversions
    from: obj.stub(:message => value)
      to: allow(obj).to receive_messages(:message => value)
```

For more details: https://github.com/yujinakayama/transpec#supported-conversions
This wasn't here before but I'm not sure why it shouldn't be?