Yeah, don't do it like that!
The rocket guys use releasable parachutes all the time, but if a deployment fails (and they do) the rocket destroys itself into the ground near the launch site. I've seen a video of this happening and it made a pretty big hole in the ground. With a balloon the landing spot could be anywhere, and a failed chute is going to see your payload meet the ground (or whoever/whatever happens to be in the way) quite quickly.
Failures can happen for lots of reasons - mechanism jamming, software error, problem with the Pi, or the GPS not reporting the altitude (you'll get this above 18km if you don't use the right type of GPS and in some cases if you don't put it into "flight mode").
Instead, do what everyone else does - use an inline chute. This will (no ifs, no buts, *will*) open automatically, within a few seconds of the balloon bursting. You can buy a specially made 'chutes from http://randomsolutions.co.uk/Random_Aer ... hutes.html
, and these have a hole in the top with short lengths of tape sewn in. You tie the balloon to these, and you hang the payload below.