If this is a Rails app, you shouldn't need to require 'rubygems'.
I don't know how else to diagnose your problem with the information given, but it's strange that it mentions app/models/feedentry.rb in the error. Since this doesn't appear to be a specifically Feedzirra related problem, you might have better luck in a general Rails forum. I would include more exact code if you do that.
-S
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