travis: sphinx-build as part of the build

Signed-off-by: Yoan Blanc <yoan.blanc@cern.ch>
docs: fix non-local images warning

Signed-off-by: Yoan Blanc <yoan.blanc@cern.ch>
docs: many fixes

Signed-off-by: Yoan Blanc <yoan.blanc@cern.ch>
Merge pull request #25 from greut/travis-sphinx

travis: sphinx-build as part of the build