Change max value of hosting_url to 255

The current value (256) makes the size of the row to big for MySQL InnoDB engine, resulting in this error:

django.db.utils.DatabaseError: Specified key was too long; max key length is 767 bytes

As the RFC (http://www.ietf.org/rfc/rfc1035.txt) fix this value to 255 as bigger, and it is enough to avoid the issue with InnoDB, it would be great to change it.

Thank you in advance.