Add explicitly supported nvim version; CI tests

[aaronik]

There have been a couple instances now of somebody on an earlier version of nvim than I'm on, whose setup broke because I updated some code based on deprecation notices I'm seeing in my editor. But those notices come from the version of nvim I'm using locally. So other setups are missed.

I'd love to get into CI the process of running the tests against various versions of neovim. That should help alleviate this issue.