If you're in a hurry, do this and you'll get "ç" instead of "ć":
Press "AltGr + ," then "c".
If you want to know a bit more, keep on reading.
This may be solved already by using one of the answers above, but I realized the best thing to do is use the keyboard layout, instead of changing things you'll have to eventually change again in the future (after updates, for example).
Before I start, keep in mind I'm using Ubuntu 14.04.2, which is not the same distro as the original question mentions (11.04). Anyway, I believe most users migrated to newwer versions by now. So:
$ cat /etc/issue
Ubuntu 14.04.2 LTS \n \l
Okay, the first thing I did was looking at the immodules files:
At the header, those files clearly mention they should not be changed, since they're generated automatically:
# GTK+ Input Method Modules file
# Automatically generated file, do not edit
# Created by /usr/lib/x86_64-linux-gnu/libgtk-3-0/gtk-query-immodules-3.0 from gtk+-3.10.8
So changing them, although it may solve the problem temporarily, is not ideal.
Looking around, I found the best answer ever about why we get a "ć" instead of a "ç" when typing ' + c: because we're really putting an acute accent on the top of letter "c". So the layout is right. With that in mind, how would one put a "kind of a" comma at the bottom of the "c" letter? Using a comma, of course!
So, the solution was the key combination AltGR + , and then "c".
No need for changing any configuration on your computer.