I believe the passage is completely literal - and it is referring to the signs of the apostles, that only the apostles can do. We have an actual fulfillment in this passage:

Acts 28:3-6 And when Paul had gathered a bundle of sticks, and laid them on the fire, there came a viper out of the heat, and fastened on his hand. And when the barbarians saw the venomous beast hang on his hand, they said among themselves, No doubt this man is a murderer, whom, though he hath escaped the sea, yet vengeance suffereth not to live. And he shook off the beast into the fire, and felt no harm. Howbeit they looked when he should have swollen, or fallen down dead suddenly: but after they had looked a great while, and saw no harm come to him, they changed their minds, and said that he was a god.

It is not a command to take up serpents, but a promise of protection in the Lord's service.