TST: Adjust boxplot tests following MPL API change

fliers on a boxplot are now 1 object instead of 2. [See here](https://github.com/matplotlib/matplotlib/issues/3544#issuecomment-57360910).

Takes care of one part of https://github.com/pydata/pandas/issues/8402
use ``mpl.__version__ >= LooseVersion('1.4')``