Move hashdiff require to class that's using it

This third-party gem is used by ConfigurationDiffer not Configure.

Also remove a stray newline from the class that it's going to.
Merge pull request #77 from alphagov/move_hashdiff_require

Move hashdiff require to class that's using it
Thank contributors

We wouldn't normally put documentation in the CHANGELOG but this came from an external contributor so we want to thank them.
Merge pull request #79 from alphagov/thanking-contributors

Thank contributors
Rename Rubocop

As of version 0.23.0[1], Rubocop is now RuboCop. This broke our CI
builds.

Fix the namespacing in our Rakefile to reflect this and use pessimistic
versioning to pin Rubocop to the 0.23.x series in our gemspec file.
Since RuboCop's API is not yet stable (as would be the case from version
1.0.0), future changes such as these could break our builds again.

[1]:
https://github.com/bbatsov/rubocop/commit/7ee7a8b47314bc067f08622d621fd9c7505dd422
Merge pull request #80 from alphagov/72432612-fix-rubocop

[#72432612] Rename Rubocop
Add some rdoc comments to ConfigurationDiffer

I double-took at the method which returns an unmodified object. Add a
comment to make it clearer why it's there.