Incorrectly parse versions of some apps

[vstoykov]

I saw that for django-celery-beat and django-celery-results it shows extra dots at the end of the version:

django_celery_beat	Django_Celery_Beat	1.0.1..
django_celery_results	Django_Celery_Results	1.0.1..

This is because both apps use custom namedtuple:

version_info_t = namedtuple('version_info_t', (
    'major', 'minor', 'micro', 'releaselevel', 'serial',
))

When releaselevel and serial are not set they are empty strings. This will lead to joining dots in the string representation.

I'm preparing a fix and will create a PR.