Turns out the last test was failing not because of a bug in NILMTK but
because something odd changed in the way DataFrames get pickled into
HDF5.  The only change I made to fix the problem was to re-create our
synthetic datasets (in nilmtk/data) using Pandas 0.14.1 and all is
fine again.  See https://github.com/pydata/pandas/issues/7748