You could try updating the composer:
sudo composer self-update
If that doest works remove composer files & then use: SSH into terminal & type :
$ cd ~
$ sudo curl -sS https://getcomposer.org/installer | sudo php
$ sudo mv composer.phar /usr/local/bin/composer
$ sudo ln -s /usr/local/bin/composer /usr/bin/composer
If you face an error that says: PHP Fatal error: Uncaught exception 'ErrorException' with message 'proc_open(): fork failed - Cannot allocate memory' in phar
/bin/dd if=/dev/zero of=/var/swap.1 bs=1M count=1024
/sbin/mkswap /var/swap.1
/sbin/swapon /var/swap.1
To install package use:
composer global require "package-name"