Go out of our way to make it clear that non-AR integrations aren't supported.

This work is to support a single project that's very bound to
ActiveRecord. Feel free to cherry-pick the AR-specific commits as you'd
like, but I'm unable to support any other integrations.
Switches to WWTD for test running; I was unable to get Appraisal running at all (bundler-within-bundler problems). Cuts down the test combinations down to the ones that affect us.
Hacks in an I18n.reload! so we can move on; I've given up on figuring out how to get the test suite to handle global-I18n-loading gracefully.