Adopt newstyle dependencies; fix travis to suit.

[pp-mo]

This is basically the content of #226 , which it aims to replace.
except ...

• rebased onto latest master
• fixed .travis.yml to get it all working (I hope)
• removed (for now) the code to test against latest-Iris instead of latest-release : just to simplify the PR

You can compare the .travis.yml directly with the one from #226
: trexfeathers/iris-grib@iris-latest...pp-mo:new_deps#diff-354f30a63fb0907d4ad57269548329e3

The whole of this doesn't compare well with #226, owing to changes since.
But the things that this PR affects are intelligible, essentially unchanged from #226 and can be compared to master (i.e. in "files changed")

[pp-mo]

When we have this agreed + merged, I will re-introduce the test-against-Iris-master code.
We will get that running + testing, but possibly in a feature-branch as to work it needs test changes which will then break testing with the old Iris release.

[coveralls]

Coverage remained the same at 92.482% when pulling c29acfb on pp-mo:new_deps into 6e2e65d on SciTools:master.

[trexfeathers]

Looks good @pp-mo

Just one question outstanding from my own PR

[pp-mo]

Re:

Is it appropriate to include wheel in here too?

In my understanding, the wheel package is only required if you want to build wheels.
cf. https://github.com/pypa/wheel

So, I think this would only be needed if you need to do a "setup.py bdist_wheel",
thus, no reason to require it here.