We've removed support for SDL 1.2 quite a while ago already, so let's
use SDL 2 now in Travis to get test coverage for SDL again.
And while we're at it, also add libsdl2-image-dev which can be used
by QEMU nowadays, too.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20191009170701.14756-3-thuth@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>