$[] is a bashism and does not work on POSIX shells

enable/disable on links numbered 08 and 09 gives the error:
'bash: 100-08: value too great for base (error token is "08")'

Signed-off-by: Einar Gunnarsson <tolvupostur@gmail.com>
---
 update-rc.d | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/update-rc.d b/update-rc.d
index c9355d7..24ac752 100644
--- a/update-rc.d
+++ b/update-rc.d
@@ -114,7 +114,7 @@ renamelink()
         # or stop link to start link with a sequence number equal to the difference of 100 minus the original sequence number.
        if ls ${etcd}${lev}.d/${oldstartstop}*${bn} >/dev/null 2>&1; then
                oldnn=`basename ${etcd}${lev}.d/${oldstartstop}*${bn}|cut -c2-3`
-               newnn=$[100-$oldnn]
+               newnn=$((100-${oldnn#0}))
                [ $verbose -eq 1 ] && echo "rename ${etcd}${lev}.d/${oldstartstop}${oldnn}${bn} -> ${etcd}${lev}.d/${newstartstop}${newnn}${bn}"
                if [ $notreally -eq 0 ];then
                        mv ${etcd}${lev}.d/${oldstartstop}${oldnn}${bn} ${etcd}${lev}.d/${newstartstop}${newnn}${bn}
--
2.17.1