Today I try to install specific laravel version with composer create-project laravel/laravel=5.1.8 your-project-name --prefer-dist, because some of the plugins have
you can do:
composer create-project laravel/laravel myproject --prefer-dist v5.1.8
To see available versions, you can visit its packagist page (lower right):
https://packagist.org/packages/laravel/framework
UPDATED:
There is no actually a tag for v5.1.8 for package laravel/laravel, Your issue is the framework(core) of laravel..
What you can do to solve it is to edit your composer.json:
"require": {
"php": ">=5.5.9",
"laravel/framework": "5.1.8"
},
And do composer update "laravel/framework"