install: map debian_version 9.X to debian stretch

Signed-off-by: Felix Abecassis <fabecassis@nvidia.com>
(cherry picked from commit ac3d74b0b4ca31d757f9c458dad3387925f8734c)
Signed-off-by: Victor Vieux <victorvieux@gmail.com>

hack/install.sh

@@ -113,6 +113,9 @@ check_forked() {
 				lsb_dist=debian
 				dist_version="$(cat /etc/debian_version | sed 's/\/.*//' | sed 's/\..*//')"
 				case "$dist_version" in
+					9)
+						dist_version="stretch"
+					;;
 					8|'Kali Linux 2')
 						dist_version="jessie"
 					;;