Use more concise declaration of DEFAULT_CACHE

In response to @acnalesso’s notes on
e3f86c8051, using ||= to make assignment
of Impressionable::ClassMethods::DEFAULT_CACHE more concise, avoiding a
call to #defined? when checking to see if the constant has been
declared.

app/models/impressionist/impressionable.rb

@@ -5,13 +5,11 @@ module Impressionist
     module ClassMethods
       attr_accessor :impressionist_cache_options
 
-      unless defined?(DEFAULT_CACHE)
-        DEFAULT_CACHE = {
+      DEFAULT_CACHE ||= {
         :counter_cache => false,
         :column_name => :impressions_count,
         :unique => false
       }
-      end
 
       def impressionist_counter_cache_options
         @impressionist_cache_options ||= {}