travis.yml: Use newer version of libgnutls and libpng

libgnutls-dev and libpng12-dev are not available in newer versions
of Ubuntu anymore, so installing these packages fails e.g. in the
new arm64 containers on Travis. Let's use newer versions of these
packages by default instead. (The old versions still get tested in
the "gcc-9" build).

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20191009170701.14756-4-thuth@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>

.travis.yml

@@ -29,7 +29,7 @@ addons:
       - libcap-dev
       - libcap-ng-dev
       - libgcc-4.8-dev
-      - libgnutls-dev
+      - libgnutls28-dev
       - libgtk-3-dev
       - libiscsi-dev
       - liblttng-ust-dev
@@ -37,7 +37,7 @@ addons:
       - libnfs-dev
       - libnss3-dev
       - libpixman-1-dev
-      - libpng12-dev
+      - libpng-dev
       - librados-dev
       - libsdl2-dev
       - libsdl2-image-dev