ubuntu - low disk space the volume boot has only X MB disk space remaining

i started getting warning popups telling me i only had 10 MB left in my /boot partition:
low disk space the volume boot has only 10 MB disk space remaining

so i first checked to see what was in my /boot folder:
ls -la /boot


which gave this output:

-rw-r--r--  1 root root   918917 mai    1  2013 abi-3.8.0-19-generic
-rw-r--r--  1 root root   918917 mai   15 00:42 abi-3.8.0-21-generic
-rw-r--r--  1 root root   919039 mai   16 17:42 abi-3.8.0-22-generic
-rw-r--r--  1 root root   919604 aug.  13 22:10 abi-3.8.0-29-generic
-rw-r--r--  1 root root   919661 aug.  22 23:21 abi-3.8.0-30-generic
-rw-r--r--  1 root root   919745 sep.  10 22:29 abi-3.8.0-31-generic
-rw-r--r--  1 root root   919745 okt.   2 01:00 abi-3.8.0-32-generic
-rw-r--r--  1 root root   919810 okt.  23 11:42 abi-3.8.0-33-generic
-rw-r--r--  1 root root   154942 mai    1  2013 config-3.8.0-19-generic
-rw-r--r--  1 root root   154942 mai   15 00:42 config-3.8.0-21-generic
-rw-r--r--  1 root root   154943 mai   16 17:42 config-3.8.0-22-generic
-rw-r--r--  1 root root   154960 aug.  13 22:10 config-3.8.0-29-generic
-rw-r--r--  1 root root   154960 aug.  22 23:21 config-3.8.0-30-generic
-rw-r--r--  1 root root   154960 sep.  10 22:29 config-3.8.0-31-generic
-rw-r--r--  1 root root   154961 okt.   2 01:00 config-3.8.0-32-generic
-rw-r--r--  1 root root   154961 okt.  23 11:42 config-3.8.0-33-generic
drwxr-xr-x  5 root root     1024 nov.   9 10:24 grub
-rw-r--r--  1 root root 16797726 mai   22 14:06 initrd.img-3.8.0-19-generic
-rw-r--r--  1 root root 16797707 mai   22 14:57 initrd.img-3.8.0-21-generic
-rw-r--r--  1 root root 16798859 mai   24 10:12 initrd.img-3.8.0-22-generic
-rw-r--r--  1 root root 16863423 aug.  22 10:09 initrd.img-3.8.0-29-generic
-rw-r--r--  1 root root 16863744 sep.   7 16:52 initrd.img-3.8.0-30-generic
-rw-r--r--  1 root root 16865732 okt.  10 08:46 initrd.img-3.8.0-31-generic
-rw-r--r--  1 root root 16868285 okt.  31 11:58 initrd.img-3.8.0-32-generic
-rw-r--r--  1 root root 16868895 nov.   9 10:24 initrd.img-3.8.0-33-generic
drwxr-xr-x  2 root root    12288 mai   22 13:48 lost+found
-rw-r--r--  1 root root   176764 des.   5  2012 memtest86+.bin
-rw-r--r--  1 root root   178944 des.   5  2012 memtest86+_multiboot.bin
-rw-------  1 root root  3060094 mai    1  2013 System.map-3.8.0-19-generic
-rw-------  1 root root  3060094 mai   15 00:42 System.map-3.8.0-21-generic
-rw-------  1 root root  3060454 mai   16 17:42 System.map-3.8.0-22-generic
-rw-------  1 root root  3062491 aug.  13 22:10 System.map-3.8.0-29-generic
-rw-------  1 root root  3062704 aug.  22 23:21 System.map-3.8.0-30-generic
-rw-------  1 root root  3062541 sep.  10 22:29 System.map-3.8.0-31-generic
-rw-------  1 root root  3062477 okt.   2 01:00 System.map-3.8.0-32-generic
-rw-------  1 root root  3062707 okt.  23 11:42 System.map-3.8.0-33-generic
-rw-------  1 root root  5356528 mai    1  2013 vmlinuz-3.8.0-19-generic
-rw-------  1 root root  5356592 mai   15 00:42 vmlinuz-3.8.0-21-generic
-rw-------  1 root root  5356688 mai   16 17:42 vmlinuz-3.8.0-22-generic
-rw-------  1 root root  5360272 aug.  13 22:10 vmlinuz-3.8.0-29-generic
-rw-------  1 root root  5361936 aug.  22 23:21 vmlinuz-3.8.0-30-generic
-rw-------  1 root root  5362320 sep.  10 22:29 vmlinuz-3.8.0-31-generic
-rw-------  1 root root  5363184 okt.   2 01:00 vmlinuz-3.8.0-32-generic
-rw-------  1 root root  5364816 okt.  23 11:42 vmlinuz-3.8.0-33-generic

so i could see that the only kernel i wanted to retain was 3.8.0-33-generic, so i removed all the old kernels like this:
sudo apt-get purge linux-image-3.8.0-{19,21,22,29,30,31,32}-generic

which gave this output:
The following packages will be REMOVED:
  linux-image-3.8.0-19-generic* linux-image-3.8.0-21-generic* linux-image-3.8.0-22-generic* linux-image-3.8.0-29-generic* linux-image-3.8.0-30-generic* linux-image-3.8.0-31-generic*
  linux-image-3.8.0-32-generic* linux-image-extra-3.8.0-19-generic* linux-image-extra-3.8.0-21-generic* linux-image-extra-3.8.0-22-generic* linux-image-extra-3.8.0-29-generic*
  linux-image-extra-3.8.0-30-generic* linux-image-extra-3.8.0-31-generic* linux-image-extra-3.8.0-32-generic*