Update demo.py

Update is needed from 'pd.Factor' attribute to 'pd.Categorical' since its how this function is called in the new version of Pandas.
Source on SO: http://stackoverflow.com/questions/21689423/pandas-attribute-error-no-attribute-factor-found
Merge pull request #20 from tomaskazemekas/patch-1

Update demo.py with change from pd.Factor to pd.Categorical