Remove 'socket' example PostgreSQL parameter

The ActiveRecord PostgreSQL adapter does not have a 'socket' option;
see:

https://github.com/rails/rails/blob/4-1-2/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb#L20
When assigning an unassigned issue send notification with the correct text.
Fix tests because previous assignee didn't exist.
Remove unecessary return.
Merge branch 'assignement_change_notifications' into 'master'

Assignment change notifications

Fixes #1377
Merge branch 'no_pg_socket' into 'master'

Remove 'socket' example PostgreSQL parameter