I have box with 1GB.

I tried increasing the memory limit to 1000M and unlimited -1 but I still get the error.

Here is the complete stack trace:
PHP Fatal error: Uncaught exception 'ErrorException' with message 'proc_open(): fork failed - Cannot allocate memory' in phar:///usr/local/bin/composer/vendor/symfony/console/Symfony/Component/Console/Application.php:974
Stack trace:
#0 [internal function]: Composer\Util\ErrorHandler::handle(2, 'proc_open(): fo...', 'phar:///usr/loc...', 974, Array)
#1 phar:///usr/local/bin/composer/vendor/symfony/console/Symfony/Component/Console/Application.php(974): proc_open('stty -a | grep ...', Array, NULL, NULL, NULL, Array)
#2 phar:///usr/local/bin/composer/vendor/symfony/console/Symfony/Component/Console/Application.php(784): Symfony\Component\Console\Application->getSttyColumns()
#3 phar:///usr/local/bin/composer/vendor/symfony/console/Symfony/Component/Console/Application.php(745): Symfony\Component\Console\Application->getTerminalDimensions()
#4 phar:///usr/local/bin/composer/vendor/symfony/console/Symfony/Component/Console/Application.php(675): Symfony\Component\Console\Application->getTerminalWidth()
#5 phar:///usr/local/bin/com in phar:///usr/local/bin/composer/vendor/symfony/console/Symfony/Component/Console/Application.php on line 974

Fatal error: Uncaught exception 'ErrorException' with message 'proc_open(): fork failed - Cannot allocate memory' in phar:///usr/local/bin/composer/vendor/symfony/console/Symfony/Component/Console/Application.php on line 974

ErrorException: proc_open(): fork failed - Cannot allocate memory in phar:///usr/local/bin/composer/vendor/symfony/console/Symfony/Component/Console/Application.php on line 974

Call Stack:
0.0005 225112 1. {main}() /usr/local/bin/composer:0
0.0120 400944 2. require('phar:///usr/local/bin/composer/bin/composer') /usr/local/bin/composer:25
0.0747 3362656 3. Composer\Console\Application->run() phar:///usr/local/bin/composer/bin/composer:43
0.0817 3677232 4. Symfony\Component\Console\Application->run() phar:///usr/local/bin/composer/src/Composer/Console/Application.php:83
16.6881 124732376 5. Composer\Console\Application->renderException() phar:///usr/local/bin/composer/vendor/symfony/console/Symfony/Component/Console/Application.php:133
16.6890 124733616 6. Symfony\Component\Console\Application->renderException() phar:///usr/local/bin/composer/src/Composer/Console/Application.php:205
16.6890 124734064 7. Symfony\Component\Console\Application->getTerminalWidth() phar:///usr/local/bin/composer/vendor/symfony/console/Symfony/Component/Console/Application.php:675
16.6891 124734216 8. Symfony\Component\Console\Application->getTerminalDimensions() phar:///usr/local/bin/composer/vendor/symfony/console/Symfony/Component/Console/Application.php:745
16.6891 124734312 9. Symfony\Component\Console\Application->getSttyColumns() phar:///usr/local/bin/composer/vendor/symfony/console/Symfony/Component/Console/Application.php:784
16.6891 124736272 10. proc_open() phar:///usr/local/bin/composer/vendor/symfony/console/Symfony/Component/Console/Application.php:974
16.6892 124737432 11. Composer\Util\ErrorHandler::handle() phar:///usr/local/bin/composer/vendor/symfony/console/Symfony/Component/Console/Application.php:974