We can run:
php -d memory_limit=-1 $(which composer) update
Or change the php.ini settings:
php -i | grep php.ini
<!-- Then we can see something like below: -->
Configuration File (php.ini) Path => /usr/local/php/etc
Loaded Configuration File => /usr/local/etc/php54/php.ini
Then change memory_limit variable:
; Maximum amount of memory a script may consume (128MB)
; http://php.net/memory-limit
memory_limit = -1