Normally I do install Laravel 5.1 by following this command regarding to documentation:
composer create-project laravel/laravel --prefer-dist
The Composer Assets Plugin you've installed locally is using a deprecated method of Composer. The plugin is already fixed, so run composer global update to get the latest versions with the bug fix. After it, you should be able to run the command succesfully.
If this doesn't work (as you might get the same error running the previous command), try removing the global vendor directory. When running any global Composer command, it outputs something like "Changed current directory to XXX". Remove the XXX/vendor directory and then try running the command.