update deleteColumn() to dropColumn() to resolve issue #442

Updated incorrect method call in [Database Abstraction Layer](http://docs.phalconphp.com/en/latest/reference/db.html#altering-tables). Correct API is documented in http://phalcon-php-framework-documentation.readthedocs.org/en/latest/api/Phalcon_Db_Adapter_Pdo_Mysql.html.