Add type field to VersionSerializer

Since the `type` wasn't in the response when syncing version and
retrieving this data the `APIVersion.type` was returning `UNKNOWN`
and then the `APIVersion.commit_name` was incorrect.

By adding the `type` to the API response the `Version` it's completed.

#3203

readthedocs/restapi/serializers.py

@@ -73,6 +73,7 @@ class Meta(object):
             'identifier', 'verbose_name',
             'active', 'built',
             'downloads',
+            'type',
         )