Readthedocs builds are failing

[djhoese]

Describe the bug
Readthedocs builds for satpy are failing with:

Installed /home/docs/checkouts/readthedocs.org/user_builds/satpy/envs/latest/lib/python3.5/site-packages/pandas-0.23.4-py3.5-linux-x86_64.egg
error: numpy 1.11.0 is installed but numpy>=1.12 is required by {'xarray'}

Wondering if we need to add a requirements.txt back in or name it something else and point rtd directly to it.

[mraspaud]

I looked a bit at this, and it seems the failures started happening after the release of xarray 10.8.
The requirements of xarray clearly state that numpy at least 1.12 is required, so I don't get why it isn't install as a dependency. I looks like numpy 1.11 is preinstalled, but that should be a problem in theory.

[djhoese]

Do you have enough information to file a bug with rtd?

[mraspaud]

I think so