Use private attr_reader in Arel

No longer needed workaround for Ruby 2.2 "private attribute?" warning.

Related 6d63b5e4.

activerecord/lib/arel/collectors/composite.rb

@@ -24,8 +24,7 @@ def value
         [left.value, right.value]
       end
 
-      protected
-
+      private
         attr_reader :left, :right
     end
   end

activerecord/lib/arel/collectors/substitute_binds.rb

@@ -21,8 +21,7 @@ def value
         delegate.value
       end
 
-      protected
-
+      private
         attr_reader :quoter, :delegate
     end
   end

activerecord/lib/arel/table.rb

@@ -104,8 +104,7 @@ def able_to_type_cast?
       !type_caster.nil?
     end
 
-    protected
-
+    private
       attr_reader :type_caster
   end
 end