Replace deprecated function

[BioGeek]

pd.set_option('display.mpl_style', 'default') throws a `FutureWarning:

/srv/venv2/local/lib/python2.7/site-packages/IPython/core/interactiveshell.py:2882: FutureWarning: 
mpl_style had been deprecated and will be removed in a future version.
Use `matplotlib.pyplot.style.use` instead.

  exec(code_obj, self.user_global_ns, self.user_ns)

So, use plt.style.use('default') instead.

[BioGeek]

Duplicate of #53 and #49