use release number instead of codename

--- a/manifests/helper/shell.pp
+++ b/manifests/helper/shell.pp
@@ -4,8 +4,8 @@
 #   include gitlabrunner::helper::shell
 class gitlabrunner::helper::shell {
 
-  case $::facts['os']['distro']['codename'] {
-    'xenial': {
+  case $::facts['os']['release']['major'] {
+    '16.04': {