look for colormap in rcParams['axes.prop_cycle'] (mpl 1.5+) first
Fixes #11614

Thanks!

Could you add a test and a release note? The test should ensure that the colors of the bar patches are correct.
 Added a new test with both a passed colormap and one set through mpl's rcParams. What do I need to do to add a release note?
 For the release note add a line in `doc/source/whatsnew/v0.18.0` under the bug fixes heading.
 I think it's good to go, though looks like I forgot to pull in the most recent changes to the release notes before adding my change... oops. Let me know if there's anything else that needs doing.
 @rcarneva Can squash the commits? (http://pandas-docs.github.io/pandas-docs-travis/contributing.html#combining-commits) For the rest this looks good I think!
 Thanks for the help with this, @TomAugspurger and @jorisvandenbossche 
 I'm going to merge this now. I think it has the same problem that all of our compatibility code for mpl 1.5 has: what if the user has a prop_cycle where color isn't in the cycler? May need to look into that if it's something matplotlib tries to support.

Thanks @rcarneva!
