re: superclass/rails3 I've overriden `retrieve_connection_pool` ;( and removed unneeded code (with superclass)

**Before**
```ruby
# Rails 4
      def retrieve_connection_pool(klass)
        class_to_pool[klass.name] ||= begin
          until pool = pool_for(klass)
            klass = klass.superclass
            break unless klass <= Base
          end

          class_to_pool[klass.name] = pool
        end
      end

# Rails 3 & 2
      def retrieve_connection_pool(klass)
        pool = @connection_pools[klass.name]
        return pool if pool
        return nil if ActiveRecord::Base == klass
        retrieve_connection_pool klass.superclass
      end
```

**After**
```ruby
ActiveRecord::ConnectionAdapters::ConnectionHandler.class_eval do
  if ActiveRecord::VERSION::MAJOR >= 4
    def retrieve_connection_pool(klass)
      class_to_pool[klass.connection_pool_name] ||= pool_for(klass)
    end
  else
    def retrieve_connection_pool(klass)
      (@class_to_pool || @connection_pools)[klass.connection_pool_name]
    end
  end
end
```