py-djangocms-picture: updated to 2.1.3

2.1.3:
Added missing migration for Picture model

2.1.2:
Fixed an issue creating a validation error on the alt attribute
Fixed an issue in the template adding a } after the srcset
Adapted test matrix for django CMS 3.4, 3.5, 3.6 as well as Django 1.11, 2.0 and 2.1
Exclude tests folder from release build

(adam)