Update .travis.yml
Update .travis.yml

pin sphinx version to prevent any future unexpected breakage
spacetelescope/understanding-json-schema,spacetelescope/understanding-json-schema,spacetelescope/understanding-json-schema,spacetelescope/understanding-json-schema,mdboom/understanding-json-schema,mdboom/understanding-json-schema,mdboom/understanding-json-schema,mdboom/understanding-json-schema