Test against latest conda-forge iris.

[pp-mo]

WIP
Test have been failing since it tests against iris-master, but was also relying on "iris/minimal-conda-requirements.txt", which is now defunct.
We "ought" to be testing against the latest Iris release, not master, in any case.
I think this was not doing so as it predated our first usable Iris 2 release.

Hopefully we now have a usable Iris v2 on conda-forge, so we should now test against that.

[DPeterK]

@pp-mo the code changes look ideal (though, as discussed offline, we'll probably need to add more packages to the install list) and we've moved the testing error onto a failure at least, so things are looking up! 🙄

[coveralls]

Coverage remained the same at 83.689% when pulling 068ad78 on pp-mo:use_iris_release into 69f4fa3 on SciTools:master.

[pp-mo]

looking up 🙄

Well it now passes !

I think this is "close", but there are some irritating dependency problems which here I'm working around with some pins ...

• cartopy 0.15 --> 0.16 broke Iris coord_systems.Stereographic.as_cartopy_crs : fixed in latest Iris master, but not released yet : see Fix for changed API of cartopy.crs.Stereographic iris#2971
• numpy 1.13 --> 1.14 broke Iris assertCML : see https://github.com/SciTools/iris/pull/3019/files#r188917443

All this should get fixed in Iris 2.1, so I think we can wait for that rather than trying to reinvent the wheel

[DPeterK]

@pp-mo nice work - this looks fine (and the tests are passing! 💯) Need to remember to treat this as an intermediary and unpin when Iris 2.1 is released, but there's no point in reinventing the wheel, as you said.