travis: Install gettext

msgmerge(1) and friends are required to build libvirt, so the
corresponding package should be installed in the Travis worker.
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>

.travis.yml

@@ -11,6 +11,7 @@ addons:
       - autopoint
       - dnsmasq-base
 #     - dwarves
+      - gettext
       - libapparmor-dev
       - libaudit-dev
       - libavahi-client-dev