debian: Bump Standards-Version to 3.9.6

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
Do not ship bats anymore.

Debian has a bats package, use it.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
.travis.yml: Install bats in Travis too

Since the Travis environment does not have access to the bats Debian
package (at least, not easily), install bats from source from
.travis.yml, before running the tests.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>