debian: bump required debhelper version.

Fixes package-needs-versioned-debhelper-build-depends lintian warning.

debian/control

diff --git a/debian/control b/debian/control
index da9c683..45c4fb7 100644
--- a/debian/control
+++ b/debian/control
@@ -2,7 +2,7 @@
 Section: httpd
 Priority: optional
 Maintainer: Sergey Budnevitch <sb@nginx.com>
-Build-Depends: debhelper (>= 7.0.50~),
+Build-Depends: debhelper (>= 9~),
                libgd2-xpm-dev,
                libgeoip-dev,
                libpcre3-dev,