This commit provides two small fixes.
Gif images are not supported by latexpdf. There are a handful of options
to resolve that issue, but the already-available imgconverter extension
takes care of converting them to supported image formats.
The pdf build was failing due to the deprecated Sphinx `app.warn` used
in the sphinx_tabs extension. The `app.log` deprecation was already
handled in this function, and now `warn` is as well.
