Fix flaky emails spec.

These tests were failing randomly because there is no guarantee
the methods `Budget#email_selected` and `Budget#email_unselected` will
always send the emails in the same order, because `investments.selected`
and `investments.unselected` don't necessarily return the records in the
order they were created.

Ordering by id is certainly not very elegant; however, ordering by
another field like `created_at` is dangerous because the record could be
created at (almost) the exact same time.

Related to issue #2446 and issue #2519.

app/models/budget.rb

@@ -152,13 +152,13 @@ class Budget < ActiveRecord::Base
   end
 
   def email_selected
-    investments.selected.each do |investment|
+    investments.selected.order(:id).each do |investment|
       Mailer.budget_investment_selected(investment).deliver_later
     end
   end
 
   def email_unselected
-    investments.unselected.each do |investment|
+    investments.unselected.order(:id).each do |investment|
       Mailer.budget_investment_unselected(investment).deliver_later
     end
   end