That's what I thought. I've been digging into the two different code branches after asking my question.
I'd update the README slightly to make the point more clear. This sentence: "Just install the latest 3.2.x to 4.1.x version of the adapter. " wasn't entirely clear that one couldn't just use the 4.1.x version of the adapter with Rails 4.2 even with the additional note about rational versioning.
@metaskills Well that sentence says it tracks "ActiveRecord" not "Rails" so the door was still open to thinking "maybe 4.1-stable will work on 4.2 Rails" -- that's the uninitiated point of view, but I see the fault in that now.
also, "4.2.x version is only for the latest 4.2 version of Rails" makes perfect sense as stated -- that tells me I can't use the 4.2.x version on earlier versions of Rails but doesn't rule out that 4.1.x version doesn't work on 4.2 version of Rails.
hindsight, of course, is 20/20 and "just trying it" showed it wouldn't work.