Convert specs to RSpec 2.14.8 syntax with Transpec

This conversion is done by Transpec 2.1.0 with the following command:
```
transpec
```

This change means our tests consistently use the `expect` syntax. The
old `should` syntax is deprecated in Rspec 3[1].

[1]:
https://www.relishapp.com/rspec/rspec-expectations/docs/syntax-configuration

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

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

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

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

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

* 1 conversion
    from: =~ /pattern/
      to: match(/pattern/)

* 1 conversion
    from: =~ [1, 2]
      to: match_array([1, 2])

See also: https://github.com/yujinakayama/transpec#supported-conversions
Disable `should` syntax in Rspec

`should` syntax is deprecated as of Rspec version 3.0 and the previous
commit converts all of our tests to use `expect` for consistency.

To prevent regressions, this change disables `should` syntax for this
gem.
Merge pull request #40 from alphagov/72424964-transpec

[#72424964] Convert Rspec tests to use `expect` syntax