There is a version key that defines the current module's version. Projects on Drupal.org do not specify this directly, as the Drupal.org extension packager adds it when a release is created. However, this key can be important for private modules to track the release information.
Versions are expected to be single strings, such as 1.0-alpha1 and 2.0.1. You can also pass VERSION, which will resolve to the current version of Drupal core.
Drupal.org does not currently support semantic versioning for contributed projects. There is a, now postponed, policy discussion in the issue queue, which can be found at https://www.drupal.org/node/1612910.