Add locale to ActiveRecord#cache_key

Problem:  The following code will pick up the fragment for the locale it was first cached in, even if Globalize.locale changes. 

```erb
<% cache @blog_post %>
  <%= @blog_post.title %>
<% end %>
```

Making ActiveRecord#cache_key sensitive to Globalize.locale gives a workaround / resolution to the problem so that rails caching can continue to work out of the box when using Globalize.