Rattlesnakes developing stronger venom
I did find an article which mentions the venom strengthening: 
http://www.sciencedaily.com/releases/2012/06/120605102638.htm

It may be an evolutionary bonus. Snakes with the most potent venom may leave fewer survivors, particularly with something like humans who would hunt down a snake if they could find it. A snake which leaves fewer survivors may be better able to escape. It may also have more successful hunts which may have been what lead them to become poisonous to begin with.

Unlike the antibiotics arms race against bacteria, it's unlikely that antivenoms are the cause of more dangerous snakes.  

As far as "centuries from now" we really can't hypothesize whether more potent venom will continue to be a more helpful evolutionary trait. More potent venom doesn't have an infinitely continuing effectiveness. After a certain point, whatever bitten isn't any more dead with a stronger poison.