CI: update builds to include matplotlib v1.5.0 when available

[jreback]

xref #11145

• change ci/requirements
• remove checks for '0+unknown' in version checks (bug already fixed in matplotlib)

[TomAugspurger]

I added a task for cleaning up the version checks that I had to workaround for the RC. I'll be adding a few more todo items.

[jreback]

just annnonced by @tcaswell !
http://matplotlib.org/devdocs/users/whats_new.html#new-in-matplotlib-1-5

[jreback]

this is ok now

[jreback]

@TomAugspurger unless you think anything is still open on this (FYI mpl 1.5 has been running on the 3.5 build since wednesday)