执行composer install遇到错误：Your requirements could not be resolved to an installable set of packages. 这是因为不匹配composer.json要求的版本
完整错误如下：
vagrant@homestead:/usr/share/nginx/html/laravel-blog$ sudocomposerinstall
Loadingcomposerrepositorieswithpackage information
Installingdependencies (includingrequire-dev) fromlockfile
Yourrequirementscouldnot beresolvedto aninstallablesetofpackages.Problem 1- Installationrequestfor doctrine/instantiator 1.0.3 -> satisfiablebydoctrine/instantiator[1.0.3].- doctrine/instantiator 1.0.3 requiresphp ~5.3 -> yourPHPversion (7.0.3) doesnot satisfythatrequirement.Problem 2- doctrine/instantiator 1.0.3 requiresphp ~5.3 -> yourPHPversion (7.0.3) doesnot satisfythatrequirement.- phpunit/phpunit-mock-objects 2.3.0 requiresdoctrine/instantiator ~1.0,>=1.0.1 -> satisfiablebydoctrine/instantiator[1.0.3].- Installationrequestfor phpunit/phpunit-mock-objects 2.3.0 -> satisfiablebyphpunit/phpunit-mock-objects[2.3.0].提示我的PHP 7版本太高，不符合composer.json需要的版本，但是在PHP 7下应该也是可以运行的，composer可以设置忽略版本匹配，命令是：
composer install --ignore-platform-reqs
composer update --ignore-platform-reqs