Update kernel files for command naming changes

ostadmin is now ostree admin.

https://bugzilla.gnome.org/show_bug.cgi?id=684049

src/ostree/kernel/15_ostree_update

@@ -7,4 +7,4 @@ if [ -z "$version" ]; then
     exit 1
 fi
 
-ostadmin update-kernel current/ ${version}
+ostree admin update-kernel /ostree/current/ ${version}