Fix violinplot with multiple identical values

Closes #368
Implement workaround for matplotlib 1.4.2 bug

This is an unpleasant hack, but this is also a nasty bug and matplotlib
apparently won't be releasing a fix for a few months:
https://github.com/matplotlib/matplotlib/pull/3699

This closes #344
Merge pull request #377 from mwaskom/fix_matplotlib_bug

Implement workaround for matplotlib 1.4.2 bug
Merge branch 'v0.5'