travis.yml: Use libsdl2 instead of libsdl1.2, and install libsdl2-image

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>

.travis.yml

@@ -39,7 +39,8 @@ addons:
       - libpixman-1-dev
       - libpng12-dev
       - librados-dev
-      - libsdl1.2-dev
+      - libsdl2-dev
+      - libsdl2-image-dev
       - libseccomp-dev
       - libspice-protocol-dev
       - libspice-server-dev
@@ -309,7 +310,8 @@ matrix:
             - libpixman-1-dev
             - libpng12-dev
             - librados-dev
-            - libsdl1.2-dev
+            - libsdl2-dev
+            - libsdl2-image-dev
             - libseccomp-dev
             - libspice-protocol-dev
             - libspice-server-dev