Don't pass raw SQL when changing sort order (#1447)

When that code runs (ordering the index page in administrate dashboard) the
following deprecation warning is shown:

> DEPRECATION WARNING: Dangerous query method (method whose arguments are used
> as raw SQL) called with non-attribute argument(s): "[schema].[table].[column]".
> Non-attribute arguments will be disallowed in Rails 6.0. This method should
> not be called with user-provided values, such as request parameters or model
> attributes. Known-safe values can be passed by wrapping them in Arel.sql().

lib/administrate/order.rb

@@ -11,7 +11,7 @@ def apply(relation)
 
       order = "#{relation.table_name}.#{attribute} #{direction}"
 
-      return relation.reorder(order) if
+      return relation.reorder(Arel.sql(order)) if
         relation.columns_hash.keys.include?(attribute.to_s)
 
       relation