a couple of bug fixes in dependencygraph.py
a couple of bug fixes in dependencygraph.py

1. `networkx.XDigraph()` was long ago [renamed](http://networkx.lanl.gov/reference/api_0.99.html) to `MultiDiGraph`
2. `cycle_finding_demo()` was out of date
nltk/nltk,nltk/nltk,nltk/nltk