Update to sphinx 1.8.x

[luisbarrancos]

We're using sphinx 1.6.x, mainly due to a limitation with sphinxcontrib-images that was fixed recently with luisbarrancos/sphinxcontrib-images@14c404b , but css3-images still suffers from a similar issue and is blocking our upgrade path to a more recent Sphinx version.
So, replace the css3-image extension by the sphinxcontrib-images extension. This is the case only for the two small tutorials in the documentation.