Fix building of documentation in readthedocs

Currently the documentation is failing to build in Read the Docs: https://readthedocs.org/projects/ligo-raven/builds/19918349/

The issue seems to be with the wheel of ligo.skymap not working correctly. This means our documentation won't compile with new changes until fixed. We should look to the example of gwcelery which also use ligo.skymap and does not have this problem.