Using CSFLE with the Ruby driver requires ffi gem. It is not specified as a dependency of the driver since most of our users do not use FLE. Right now the driver fails with the following error if ffi is not installed, and a user tries to do FLE operations:

LoadError: cannot load such file -- ffi

We need to log a proper warning in this case, including a clear explanation what happened and how to add ffi gem to the project.