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

app/models/ahoy/data_source.rb

@@ -4,7 +4,6 @@
 
 module Ahoy
   class DataSource
-
     # Adds a collection with the datasource
     # Name is the name of the collection and will be showed in the
     # chart
@@ -43,7 +42,5 @@ module Ahoy
       def add_key(key)
         shared_keys.push(key) unless shared_keys.include? key
       end
-
   end
-
 end