create officer assignments in order

Make sure we create the Poll::OfficerAssignments in the same order the
booth_assignments where previously created. So the spec "Poll::Shift
officer_assignments creates and destroy corresponding
officer_assignments does" not fail.

app/models/poll/shift.rb

@@ -24,7 +24,7 @@ class Poll
     end
 
     def create_officer_assignments
-      booth.booth_assignments.each do |booth_assignment|
+      booth.booth_assignments.order(:id).each do |booth_assignment|
         attrs = {
           officer_id:          officer_id,
           date:                date,