Update example for recent versions of ActiveRecord

`find_by_*` was replaced with `find_by(conditions_hash)` and `find_or_initialize_by_*` was replaced with `find_or_initialize_by(conditions_hash)`

README.md

@@ -316,12 +316,12 @@ The following example implements an `ActiveRecord` store to save exchange rates
 
 class ExchangeRate < ActiveRecord::Base
   def self.get_rate(from_iso_code, to_iso_code)
-    rate = find_by_from_and_to(from_iso_code, to_iso_code)
+    rate = find_by(:from => from_iso_code, :to => to_iso_code)
     rate.present? ? rate.rate : nil
   end
 
   def self.add_rate(from_iso_code, to_iso_code, rate)
-    exrate = find_or_initialize_by_from_and_to(from_iso_code, to_iso_code)
+    exrate = find_or_initialize_by(:from => from_iso_code, :to => to_iso_code)
     exrate.rate = rate
     exrate.save!
   end