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.

Also adds a `spec_helper.rb` to accomodate the RSpec configuration,
consistent with the other gems.
Merge pull request #10 from alphagov/72424964-transpec

[#72424964] Disable `should` syntax in Rspec