Alias prev_page to previous_page for kaminari compatibility

An internal change in kaminari https://github.com/amatsuda/kaminari/commit/e07914f69c6878e357d6f44a8663fd7a9223c54b broke compatibility with sunspot, which was reported in https://github.com/amatsuda/kaminari/issues/613. This pull request fixes the issue. Let me know is there's something missing. Thanks!