If you're lucky, you can just change the version required in the "manifest.json". It's a text file, you can open it in notepad. It should look like this, you'll probably need to change the value on the MinimumAPI line.
"Name": "BetterHay",
"Author": "Cat",
"Version": "2.1.1",
"Description": "You get hay from scything grass regardless of whether you have silos, and can always remove hay from hoppers.",
"UniqueID": "cat.betterhay",
"EntryDll": "BetterHay.dll",
"MinimumApiVersion": "2.0",
"UpdateKeys": [ "Nexus:1430" ]
If that doesn't work someone with some coding knowledge will have to get around to it, eventually.