ubuntu: use any init-system-helpers

init-system-helpers (>= 1.56~) can't be satisfied on:

* Ubuntu 18.04 (1.51)
* Ubuntu 16.04 (1.29)
* Debian 9 (1.48)

Remove the specific version requirement in order to enable build on
older platforms.

distro/pkg/deb/control

@@ -21,7 +21,7 @@ Homepage: https://bird.network.cz/
 
 Package: bird2
 Architecture: kfreebsd-any linux-any
-Pre-Depends: init-system-helpers (>= 1.56~),
+Pre-Depends: init-system-helpers,
              ${misc:Pre-Depends}
 Depends: adduser,
          lsb-base,