sqlalchemy: fix default integer constructor

* Addresses issue with default constructor on types that do not
  accept arguments in SQLAlchemy>=0.9. (closes #1776)

Signed-off-by: Leonardo Rossi <leonardo.r@cern.ch>
I think `impl` is not needed according to http://docs.sqlalchemy.org/en/rel_0_9/core/types.html#sqlalchemy.types.TypeDecorator.load_dialect_impl