laravel5.5を入れて5.6へcomposer update

[vagrant@localhost laravel]$ php composer.phar create-project laravel/laravel blog “5.5.*”

[vagrant@localhost blog]$ php artisan –version
Laravel Framework 5.5.45
[vagrant@localhost blog]$ php -v
PHP 7.1.21 (cli) (built: Aug 15 2018 18:11:46) ( NTS )
Copyright (c) 1997-2018 The PHP Group
Zend Engine v3.1.0, Copyright (c) 1998-2018 Zend Technologies

composer.json

"require": {
        "php": ">=7.1.3",
        "fideloper/proxy": "~4.0",
        "laravel/framework": "5.6.*",
        "laravel/tinker": "~1.0",
        "olssonm/l5-very-basic-auth":"5.*"
    },
    "require-dev": {
        "filp/whoops": "~2.0",
        "fzaninotto/faker": "~1.4",
        "mockery/mockery": "~1.0",
        "phpunit/phpunit": "~7.0",
        "symfony/thanks": "^1.0"
    },

[vagrant@localhost blog]$ php composer.phar update
Loading composer repositories with package information
Updating dependencies (including require-dev)
Package operations: 2 installs, 22 updates, 3 removals
– Removing symfony/polyfill-php70 (v1.11.0)
The following exception is caused by a lack of memory or swap, or not having swap configured
Check https://getcomposer.org/doc/articles/troubleshooting.md#proc-open-fork-failed-errors for details

PHP Warning: proc_open(): fork failed – Cannot allocate memory in phar:///home/vagrant/local/app/laravel/blog/composer.phar/vendor/symfony/console/Application.php on line 952

Warning: proc_open(): fork failed – Cannot allocate memory in phar:///home/vagrant/local/app/laravel/blog/composer.phar/vendor/symfony/console/Application.php on line 952

[ErrorException]
proc_open(): fork failed – Cannot allocate memory

update [–prefer-source] [–prefer-dist] [–dry-run] [–dev] [–no-dev] [–lock] [–no-custom-installers] [–no-autoloader] [–no-scripts] [–no-progress] [–no-suggest] [–with-dependencies] [–with-all-dependencies] [-v|vv|vvv|–verbose] [-o|–optimize-autoloader] [-a|–classmap-authoritative] [–apcu-autoloader] [–ignore-platform-reqs] [–prefer-stable] [–prefer-lowest] [-i|–interactive] [–root-reqs] [–] []…