travis-ci: Add swig and libpython-dev to the package list

As part of 1905c8fc71 we introduced failures depending on if swig and
libpython-dev are installed or not.  To provide coverage for this are of
code in the future ensure we have these packages installed.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Heiko Schocher <hs@denx.de>

.travis.yml

@@ -19,6 +19,8 @@ addons:
     - libsdl1.2-dev
     - python
     - python-virtualenv
+    - swig
+    - libpython-dev
     - gcc-powerpc-linux-gnu
     - gcc-arm-linux-gnueabihf
     - gcc-aarch64-linux-gnu