Add pandas release 0.13 to Travis CI tests

Pandas 0.13 [has now been released](http://pandas.pydata.org/pandas-docs/stable/whatsnew.html#v0-13-0-january-1-2014), so added the pandas 0.13 tag to the matrix of tests on Travis CI.  Now running 3 different pandas versions, 0.12, 0.13 and master.