Updating iopubwatcher.py to query for 'text' instead 'data' in the message dict

It appears the message format has changed underneath this script, which I use with trivial changes to watch the progress of long-running computations.

Right now querying for `data` raises a `KeyError` for me on the current master branch.

I'm pretty ignorant of the low-level ZMQ messaging IPython uses -- please let me know if an alternative fix would be better.