Changed Django references to Flask

Flask (http://flask.pocoo.org/) is a simple, easy-to-understand web
framework for Python. It's less full-featured than Django (it's got
a few more features than Sinatra) but that is nice, as you aren't stuck
with Django's defaults, which are nigh impossible to change. For the
level of web stuff I imagine in the course, I think Flask will be a
great choice.