version-increment increments "beta11" to "beta2"

Admitted, this is an edge case - but a bug none the less. If you run drush fu --version-increment on a feature with version 7.x-1.0-beta11, it is incremented to 7.x-1.0-beta2. I'm not very good at regex, but I suspect that the regex in _drush_features_generate_export() is already satisfied when it finds a number at the last position of the string and doesn't look any further.