test against xarray master

[dcherian]

It'd be nice if satpy and other relevant pytroll projects could run CI with xarray's master branch so we can catch bugs like pydata/xarray#3746 and #1058 earlier.

We test against master branches of dask etc like this:
https://github.com/pydata/xarray/blob/4c96d53e6caa78d56b785f4edee49bbd4037a82f/ci/azure/install.yml#L13-L34

[djhoese]

As an add on to this, we should probably start failing on warnings in at least one environment (specified as an allowed failure).

[mraspaud]

@dcherian thanks for bringing this up. I think it's a good idea, we will discuss it and see how we can modify .travis.yml to include the master branches of xarray, dask, rasterio...

[djhoese]

As pointed out in the various related PRs, this has already helped point out an issue with how our trollimage project is using expand_dims and copying the data. Thanks @dcherian for suggesting it.