Switch to conda based test environment.

This should get the tests passing again on Travis-CI.

Also, by using conda it has the benefit that it's super easy to replicate the test environment on another machine just by using: `conda create -n <name> --file conda-requirements.txt`.

It's also slightly quicker ... not that you'd really notice.
NB. I excluded this directory because it needs Iris and Cartopy to run, but we could easily add iris/cartopy to the conda environment and include these tests again.