Improve generated code

As discussed on #439, the thoughtbot style guide has changed since the clearance generators were written, so the generated code no longer follows the guide and causes a lot of warnings from @houndci.

This PR fixes three categories of style guide violations, and means the generators can be run without any complaints from our faithful hound:

* Single quoted strings, where we now prefer double quoted strings.
* Missing blank lines at the end of files.
* Using RSpec's `should` syntax, where we now prefer the `expect` syntax (hound doesn't pick this up yet).

I used these PRs to check hound's reaction to the changes:

* From master (currently b2f4b240a8ee2dbb1e8679274aa188629f2e3796): https://github.com/georgebrock/clearancetest/pull/2
* From this branch: https://github.com/georgebrock/clearancetest/pull/3
Prefer single-quoted strings when you don't need string interpolation or special symbols.