I noticed when calling set_log_file that it's on the top-level namespace.
And so is the rest of the gem. It's not a good idea to do this as it
pollutes the user's application. Best practice is to wrap all code in a
module which by convention is the gem name.
I was also getting `Uninitialized constant FFI:NIL` and in the
set_log_file method and I could not tell why it was needed. I changed it
to a regular ruby nil. If it's needed for some reason I'll change it
back or move to another PR.
