Fix deprecation warning of Time#to_s

```
DEPRECATION WARNING: Time#to_s(:number) is deprecated. Please use Time#to_fs(:number) instead.
```

lib/active_hash/base.rb

@@ -487,7 +487,11 @@ def cache_key
         when new_record?
           "#{self.class.cache_key}/new"
         when timestamp = self[:updated_at]
-          "#{self.class.cache_key}/#{id}-#{timestamp.to_s(:number)}"
+          if ActiveSupport::VERSION::MAJOR < 7
+            "#{self.class.cache_key}/#{id}-#{timestamp.to_s(:number)}"
+          else
+            "#{self.class.cache_key}/#{id}-#{timestamp.to_fs(:number)}"
+          end
         else
           "#{self.class.cache_key}/#{id}"
       end

spec/active_hash/base_spec.rb

@@ -1314,7 +1314,7 @@ class Book < ActiveRecord::Base
         {:id => 1, :name => "foo", :updated_at => timestamp}
       ]
 
-      expect(Country.first.cache_key).to eq("countries/1-#{timestamp.to_s(:number)}")
+      expect(Country.first.cache_key).to eq("countries/1-#{timestamp.to_fs(:number)}")
     end
 
     it 'should use "new" instead of the id for a new record' do