Fix Ruby "method refined" warning (#433)

When Ruby warnings are enabled, this is printed when loading the
sitemap_generator gem:

> sitemap_generator-6.3.0/lib/sitemap_generator/templates.rb:16):1: warning: method redefined; discarding old sitemap_sample

This is because an attr reader (e.g. `sitemap_sample`) was being defined
via `attr_accessor` and then immediately redefined with `define_method`.

Fix by using `attr_writer` instead of `attr_accessor`, so that the attr
reader is not defined twice.

lib/sitemap_generator/templates.rb

@@ -13,7 +13,7 @@ class Templates
     }
 
     # Dynamically define accessors for each key defined in <tt>FILES</tt>
-    attr_accessor(*FILES.keys)
+    attr_writer(*FILES.keys)
 
     FILES.each_key do |name|
       eval(<<-ACCESSOR, binding, __FILE__ , __LINE__ + 1)