You already installed symfony/console globally in version 2.8.8 and the version is locked in ~/.config/composer/composer.lock. This is probably due to some older installation of a different tool. If you are lucky this is also compatible with symfony/console 3.1 which magestead needs apparently.
Try the following:
composer global require --no-update "richdynamix/magestead"
composer global update
This command will also update other installed packages. The one that you used ("composer require" with implicit update) does not update other packages that are already installed.
If you still get the error, the version conflict cannot be solved and you can't install all your tools globally. Install magestead in your project instead, or in a separate directory.
