Upgrade selfoss to version 2.15

Changelog at https://github.com/SSilence/selfoss/releases/tag/2.15

roles/news/defaults/main.yml

 selfoss_db_username: selfoss
 selfoss_db_password: "{{ lookup('password', secret + '/' + 'selfoss_db_password', length=32) }}"
 selfoss_db_database: selfoss
-selfoss_version: 2.14
+selfoss_version: 2.15
 
 selfoss_username: "{{ main_user_name }}"
 # this is the sha512 hash of the desired password