Use CURRENT_TIMESTAMP in favour of NOW() in TrashedFilter

Use of NOW() directly in TrashedFilter is MySQL specific and does not work with other database engines e.g. Oracle, SQLite etc. 

CURRENT_TIMESTAMP is a better value as it features in standard ANSI SQL and is supported by most DBMS. See:

http://stackoverflow.com/questions/20541362/db-agnostic-sql-for-current-timestamp