Make RTD builds faster

[fmaussion]

See #2306

This makes the builds a little bit faster. But we are still very close to the 900 s limit.

I tried to pin more packages this morning but this didn't work out. I'll merge this first (the latest build worked with this config, https://readthedocs.org/projects/xray/builds/) and try again at a later stage.

[shoyer]

Thanks for looking into this! I've found a pretty high variance on RTD depending on network latency. With our current setup the build completes about one in four times.

[fmaussion]

Yes, and now it comes that a C dependency is missing from the conda channels: http://xarray.pydata.org/en/fix-docs/auto_gallery/plot_rasterio_rgb.html#sphx-glr-auto-gallery-plot-rasterio-rgb-py

Will unpin everything and see if it works.

[fmaussion]

OK so I managed to get a successful build will all packages unpinned this morning, but not after pinning them back to fixed versions (although you would think this makes things faster).

Since its always a few seconds that are missing I opened readthedocs/readthedocs.org#4432 by RTD to see if we can get an extension...

[fmaussion]

See resolution here: readthedocs/readthedocs.org#4432