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: Tom Rini <trini@konsulko.com>
diff --git a/.travis.yml b/.travis.yml
index e6db9d6..11f9757 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -22,6 +22,7 @@
     - libsdl2-dev
     - python
     - python-pyelftools
+    - python3-sphinx
     - python3-virtualenv
     - python3-pip
     - swig