update command is very memory hungry
and there's no good solution for that
as the memory usage is unexpected, just disable it

https://github.com/composer/composer/issues/1898

--- composer-1.0.0-14.alpha11/src/Composer/Command/UpdateCommand.php~	2015-11-14 18:21:07.000000000 +0200
+++ composer-1.0.0-14.alpha11/src/Composer/Command/UpdateCommand.php	2015-11-25 17:46:18.524578082 +0200
@@ -75,6 +75,8 @@
 
     protected function execute(InputInterface $input, OutputInterface $output)
     {
+        ini_set('memory_limit', -1);
+
         $io = $this->getIO();
         if ($input->getOption('no-custom-installers')) {
             $io->writeError('<warning>You are using the deprecated option "no-custom-installers". Use "no-plugins" instead.</warning>');