travis.yml: Drop libcap-dev

Commit 7e462613 converted virtfs-proxy-helper to using libcap-ng. There
aren't any users of libcap anymore. No need to install libcap-dev.
Signed-off-by: NGreg Kurz <groug@kaod.org>
Reviewed-by: NThomas Huth <thuth@redhat.com>
Reviewed-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Acked-by: NAlex Bennée <alex.bennee@linaro.org>

.travis.yml

@@ -26,7 +26,6 @@ addons:
       - libaio-dev
       - libattr1-dev
       - libbrlapi-dev
-      - libcap-dev
       - libcap-ng-dev
       - libgcc-4.8-dev
       - libgnutls28-dev