[ADAM-1917] Set Sphinx version in requirements.txt.
[ADAM-1917] Set Sphinx version in requirements.txt.

Resolves #1917.

heuermh/adam,bigdatagenomics/adam,bigdatagenomics/adam,heuermh/adam,bigdatagenomics/adam,heuermh/adam,bigdatagenomics/adam,heuermh/adam