执行 Composer require "mews/captcha:~2.0" 失败了。肿么办？

vagrant@homestead:~/Code/larabbs$ composer require "mews/captcha:~2.0"
./composer.json has been updated
Loading composer repositories with package information
Updating dependencies (including require-dev)

Failed to decode response: zlib_decode(): data error
Retrying with degraded mode, check https://getcomposer.org/doc/articles/troub... for more info
Package operations: 5 installs, 0 updates, 0 removals

• Installing ralouphie/getallheaders (3.0.3): Downloading (failed)
  Downloading (failed)
  Downloading (failed)
  Failed, trying the next URL (0: proc_open(): fork failed - Cannot allocate memory)Downloading (failed)
  Downloading (failed)
  Downloading (failed)The following exception is caused by a lack of memory or swap, or not having swap configured
  Check https://getcomposer.org/doc/articles/troub... for details

PHP Warning: proc_open(): fork failed - Cannot allocate memory in phar:///usr/local/bin/composer/vendor/symfony/console/Application.php on line 959

Warning: proc_open(): fork failed - Cannot allocate memory in phar:///usr/local/bin/composer/vendor/symfony/console/Application.php on line 959

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

require [--dev] [--prefer-source] [--prefer-dist] [--no-progress] [--no-suggest] [--no-update] [--no-scripts] [--update-no-dev] [--update-with-dependencies] [--update-with-all-dependencies] [--ignore-platform-reqs] [--prefer-stable] [--prefer-lowest] [--sort-packages] [-o|--optimize-autoloader] [-a|--classmap-authoritative] [--apcu-autoloader] [--] []...