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

As part of 1905c8fc 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>