Почему не устанавливается PHP GD?

Не могу установить php-gd

Пробовал такие команды:

sudo apt-get install php5-gd
sudo apt-get install php-gd

А выдает:

Reading package lists... Done
Building dependency tree
Reading state information... Done
Package php5-gd is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Package 'php5-gd' has no installation candidate

На сервере установлена Ubuntu 14.04 и php 5.5