ENH: Set default zorder for LAND and OCEAN

[dopplershift]

This should help keep them behind most plot elements by default. This
fixes a problem where wind barbs end up hidden by these features.

[QuLogic]

Re-trigger Travis?

[dopplershift]

Travis failure is complaining about libgfortran. Can someone try restarting it?

[QuLogic]

Didn't work; did conda-forge drop support for 3.4?

[dopplershift]

That sounds familiar...though the packages that were there should still be there.

[ocefpaf]

That sounds familiar...though the packages that were there should still be there.

And that is exactly the problem. The remaining packages never got updated to defaults latest libgfortran and they are expecting an old one that probably was not pinned down at the time they were built.

I can re-build the stack for cartopy on Python 3.4 if that is really important here. If not, I recommend to just drop Python 3.4 from the tests.

[ajdawson]

Yeah we should probably just be testing 3.5 and 3.6 and drop 3.4 as @ocefpaf says

[dopplershift]

I can change 3.4 -> 3.5 and 3.5 ("latest everything") to 3.6 in .travis.yml. Does that need to be a separate PR?

[ajdawson]

Well, probably. It may be useful to establish that changing the test setup doesn't break anything independently from the changes here.

[dopplershift]

Done in #926.

[ajdawson]

Can you rebase this please @dopplershift

[dopplershift]

Done.

[ajdawson]

Thanks @dopplershift