Composerでプラグインをインストールする時のエラー

composer update

Your requirements could not be resolved to an installable set of packages.

Problem 1
- cakephp/cakephp 2.6.9 requires ext-mcrypt * -> the requested PHP extension mcrypt is missing from your system.
- cakephp/cakephp 2.6.8 requires ext-mcrypt * -> the requested PHP extension mcrypt is missing from your system.
- cakephp/cakephp 2.6.7 requires ext-mcrypt * -> the requested PHP extension mcrypt is missing from your system.
- cakephp/cakephp 2.6.6 requires ext-mcrypt * -> the requested PHP extension mcrypt is missing from your system.
- cakephp/cakephp 2.6.5 requires ext-mcrypt * -> the requested PHP extension mcrypt is missing from your system.
- cakephp/cakephp 2.6.4 requires ext-mcrypt * -> the requested PHP extension mcrypt is missing from your system.
- cakephp/cakephp 2.6.3 requires ext-mcrypt * -> the requested PHP extension mcrypt is missing from your system.
- cakephp/cakephp 2.6.2 requires ext-mcrypt * -> the requested PHP extension mcrypt is missing from your system.
- cakephp/cakephp 2.6.12 requires ext-mcrypt * -> the requested PHP extension mcrypt is missing from your system.
- cakephp/cakephp 2.6.11 requires ext-mcrypt * -> the requested PHP extension mcrypt is missing from your system.
- cakephp/cakephp 2.6.10 requires ext-mcrypt * -> the requested PHP extension mcrypt is missing from your system.
- cakephp/cakephp 2.6.1 requires ext-mcrypt * -> the requested PHP extension mcrypt is missing from your system.
- cakephp/cakephp 2.6.0 requires ext-mcrypt * -> the requested PHP extension mcrypt is missing from your system.
- cakephp/cakephp 2.6.8 requires ext-mcrypt * -> the requested PHP extension mcrypt is missing from your system.
- Installation request for cakephp/cakephp 2.6.* -> satisfiable by cakephp/cakephp[2.6.0, 2.6.1, 2.6.10, 2.6.11, 2.6.12, 2.6.2, 2.6.3, 2.6.4, 2.6.5, 2.6.6, 2.6.7, 2.6.8, 2.6.9].

 mcryptがないようと怒られる

$sudo apt-get install php5-mcrypt

再度試せばOK