bugfix: use 'ls -ult' instead of 'ls -clt'

When sorting the directories for each interval 'ls -clt' was used.
'ls -clt' would list the date of the last time the rsnapshot was ran,
not when it was initially created.

The correct command is 'ls -ult', which prints the expected information.

Example:

For a status like this:

$ ls -l | grep daily
drwxr-xr-x 3 eddy eddy 4096 mar 19 22:54 daily.0
drwxr-xr-x 3 eddy eddy 4096 mar 19 19:25 daily.1
drwxr-xr-x 3 eddy eddy 4096 mar  5 23:38 daily.2
drwxr-xr-x 3 eddy eddy 4096 mar  1 02:27 daily.3
drwxr-xr-x 3 eddy eddy 4096 feb 16 13:21 daily.4
drwxr-xr-x 3 eddy eddy 4096 feb  2 00:27 daily.5
drwxr-xr-x 3 eddy eddy 4096 feb  1 12:20 daily.6

ls -clt would print:

$ ls -clt | grep daily
drwxr-xr-x 3 eddy eddy 4096 apr  3 01:39 daily.0
drwxr-xr-x 3 eddy eddy 4096 apr  3 01:39 daily.1
drwxr-xr-x 3 eddy eddy 4096 apr  3 01:39 daily.2
drwxr-xr-x 3 eddy eddy 4096 apr  3 01:39 daily.3
drwxr-xr-x 3 eddy eddy 4096 apr  3 01:39 daily.4
drwxr-xr-x 3 eddy eddy 4096 apr  3 01:39 daily.5
drwxr-xr-x 3 eddy eddy 4096 apr  3 01:39 daily.6

But this is not what we want!
We want to compare the dates shown by default by 'ls -l'.

Here is what 'ls -ult' prints:

$ ls -ult | grep daily
drwxr-xr-x 3 eddy eddy 4096 mar 19 22:54 daily.0
drwxr-xr-x 3 eddy eddy 4096 mar 19 19:25 daily.1
drwxr-xr-x 3 eddy eddy 4096 mar  5 23:38 daily.2
drwxr-xr-x 3 eddy eddy 4096 mar  1 02:27 daily.3
drwxr-xr-x 3 eddy eddy 4096 feb 16 13:21 daily.4
drwxr-xr-x 3 eddy eddy 4096 feb  2 00:27 daily.5
drwxr-xr-x 3 eddy eddy 4096 feb  1 12:20 daily.6

Bingo!

Signed-off-by: Eddy Petrișor <eddy.petrisor@gmail.com>