Maybe your composer is outdated.
Below are the steps to get rid of the error.
Note: For Windows professionals, Only Step2 and Step3 is needed and done.
Step1
Remove the composer:
sudo apt-get remove composer
Step2
Download the composer:
php -r "copy('https://getcomposer.org/installer', 'composer-setup.php');"
Step3
Run composer-setup.php file
php composer-setup.php
Step4
Finally move the composer:
sudo mv composer.phar /usr/local/bin/composer
Your composer should be updated now. To check it run command:
composer
You can remove the downloaded composer by php command
php -r "unlink('composer-setup.php');"