travis.yml: Drop libcap-dev

Commit 7e46261368 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: Greg Kurz <groug@kaod.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Acked-by: Alex 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