travis: Add python3-sphinx to the package list

In order to build htmldocs we need sphinx-build which comes from
python3-sphinx.
Signed-off-by: NTom Rini <trini@konsulko.com>

.travis.yml

@@ -22,6 +22,7 @@ addons:
     - libsdl2-dev
     - python
     - python-pyelftools
+    - python3-sphinx
     - python3-virtualenv
     - python3-pip
     - swig