Remove support/testing for Ruby 1.8

This version of Ruby doesn't maintain the order of hashes which makes it
non-trivial to render the config hash in a way that doesn't vary on every
run. We're not using Ruby 1.8 and hopefully less other people are now. So
remove it.

---

This replaces an attempt in #4 and fixes tests in #3.

/cc @alphagov/team-infrastructure-admins