drush/drush dev-master requires codegyre/robo ~1.0.0-beta1
The dev/master branch of Drush is a development release, so it depends on non-stable components such as Robo 1.0.0-beta1. Composer will reject this component unless you have "minimum-stability": "beta" in your composer.json file.  Note that Drush itself has this setting, but if you include Drush from some other project (e.g. your Drupal site), then you will also need to set your minimum-stability in your project's top-level composer.json file.

Perhaps a more reasonable solution would be to run composer require drush/drush:~8, and use the stable release of Drush.

UPDATE

Drush master is now using the stable release of Robo ~1, so it should be easier to install Drush via composer require, at least for the time being. It is always possible that at some point in the future, though, the dev version of Drush may become dependent on features only available in some non-stable release of one of its dependencies, so you should always be prepared to set your minimum stability when requiring a dev version of Drush -- or any other Composer project.