fixed issue with fit method. Earlier was using ancestors method which used to return all the nodes above it in the family which we didn't need. Now have replaced it with predecessors
adds pandas to travis
Merge remote-tracking branch 'upstream/dev' into issue224

Conflicts:
	.travis.yml
	pgmpy/models/BayesianModel.py
Merge remote-tracking branch 'upstream/dev' into issue224
Merge remote-tracking branch 'upstream/dev' into issue224
Merge remote-tracking branch 'upstream/dev' into issue224
Merge pull request #225 from ankurankan/issue224

Fit and predict methods for Bayesian Models