Add and apply rubocop rules for empty lines

We were very inconsistent regarding these rules.

Personally I prefer no empty lines around blocks, clases, etc... as
recommended by the Ruby style guide [1], and they're the default values
in rubocop, so those are the settings I'm applying.

The exception is the `private` access modifier, since we were leaving
empty lines around it most of the time. That's the default rubocop rule
as well. Personally I don't have a strong preference about this one.


[1] https://rubystyle.guide/#empty-lines-around-bodies

lib/tasks/budgets.rake

@@ -1,6 +1,5 @@
 namespace :budgets do
   namespace :email do
-
     desc "Sends emails to authors of selected investments"
     task selected: :environment do
       Budget.last.email_selected
@@ -10,7 +9,6 @@ namespace :budgets do
     task unselected: :environment do
       Budget.last.email_unselected
     end
-
   end
 
   desc "Update investments original_heading_id with current heading_id"
@@ -23,5 +21,4 @@ namespace :budgets do
       print "."
     end
   end
-
 end