Fix Rails 6.0 deprecation warning

by making all class methods public. Previously a call to #near or
within_bounding_box produced:

  DEPRECATION WARNING: Delegating missing full_column_name method to
  Venue. Accessibility of private/protected class methods in :scope is
  deprecated and will be removed in Rails 6.0.

I don't like turning all these methods public but at the moment I don't
see any way around it.

lib/geocoder/stores/active_record.rb

@@ -166,8 +166,6 @@ module Geocoder::Store
         }
       end
 
-      private # ----------------------------------------------------------------
-
       ##
       # SQL for calculating distance based on the current database's
       # capabilities (trig functions?).