COMPAT: Support for MPL 1.5
closes https://github.com/pydata/pandas/issues/11111  (nice issue number)

Testing with tox locally I get 1 failure using either matplotlib 1.4 or 1.5. I may have messed up my tox config though. Still need to cleanup a bunch of stuff, I just threw in changes to get each test passing as I went.

release notes et. al coming tonight or tomorrow. Are we making any changes to the build matrix to run a matplotlib 1.5?

u can add to the channel list (I think I showed this above)

it will thrn pick up the latest version if it's not specified exactly
 is this not tested on travis? I don't see the icon
 Travis was flaky yesterday morning. I just pushed some changes, and it's picked up now.
 @jreback travis is green. Looks like https://travis-ci.org/pydata/pandas/jobs/81429896 and https://travis-ci.org/pydata/pandas/jobs/81429897 both ran with matplotlib RC1.
 make the 3.4 regular build not specify a matplotlib (in requirements-3.4.run). All of the slow tests are running (e.g. 2.7) just fine. But I don't think the fast ones actually are run by travis (in theory they are on 3.3, but that is not build ATM for 1.5.0rc1).
 @jreback done. We'll want to followup and pin it at 1.5 once that's released. I'm assuming the `conda-forge` channel will continue to track master and we won't want changes there breaking our builds. 
 thanks @TomAugspurger 
 @TomAugspurger I don't think we plan to update conda forge from master, just tags.
