prevent the constraint to be recreated on update

This fixes https://github.com/odoo/odoo/issues/2112 at least for move
lines. If the constraint is not written exactly like that, the next time
we update the module OpenERP does not recognize it and drops and
recreates the constraint. This is dangerous, and takes a long time in a
big table.
Merge pull request #43 from lepistone/7.0-fix-move-line-constraint

prevent the constraint to be recreated on update