Remove simplecov dependency.
Remove rr dependency.

Unused since afffb58.
Remove debugger dependency.
Remove atomic dependency.

Unused? Reported as missing in #52 (436cae2).
Swap out builder dependency (via activemodel) for nokogiri.

`nokogiri` also used by `equivalent-xml` and may replace `rexml` in the future.
Swap out activemodel dependency for equivalent-xml.
Remove activesupport dependency.

Use `unicode` or `activesupport` for Unicode support if available.
Remove support files for managing Ruby versions.

We generally default to the latest stable Ruby.