How to lock a GEM version

I’ve moved an app from one server to another, which resulted in my app
using RedCloth 3.0.4 instead of 3.0.3. This basically wreaked havoc on
my app, and since both versions are installed on my current server,
I’d like to force my Rails app to use the older one.

I looked and looked and can’t figure out how to do this. It must be
simple! Can anyone point me to the solution?