Currently experiencing issues with the latest `dask` version `2.0.0` when using Python 3.6 and 3.7 in Travis CI for `iris`, see [this Travis CI build](https://travis-ci.org/bjlittle/iris/builds/555755873). As a result, we've pinned back to `dask<2`