I'm not sure this is proof of rigging as we don't know the context of the email. People could have subitted questions they wanted asked and CNN vetted and responded back with the ones they wanted. That's not 'rigging', thats the normal nature of producing a program.
Even if they did, it doesn't sound like it hurt in any way. From what I've read, Cruz paved over Sanders last night.
Gateway Pundit seems to be doing what they do best, jumping to YUGE conclusions with little evidene.