skbio.draw.boxplots hack for empty distributions

Empty distributions are plottable in mpl < 1.4.0. In 1.4.0, a ValueError is raised. This has been fixed in mpl 1.4.0-dev (see https://github.com/matplotlib/matplotlib/pull/3571). In order for skbio.draw.boxplots to support empty distributions across mpl versions, empty distributions are replaced with [np.nan]. See https://github.com/pydata/pandas/issues/8382 and https://github.com/pydata/pandas/pull/8240 for details.
TST: remove unit test docstrings from test_distributions.py
Add tests and comments for np.nan hack
TST: travis tests against mpl < 1.4.0 and latest version
Merge branch 'master' into issue-649

Conflicts:
	setup.py
	skbio/draw/_distributions.py
TST: update dep versions for consistency

Based on @Jorge-C's suggestion
Revert "TST: update dep versions for consistency"

This reverts commit 1e12536f793b2c59878078c88447349432b67e15.
Merge pull request #700 from jairideout/issue-649

Support newest matplotlib release