Update ssn_high_group_code_loader.rb

Modify date-matching regex to handle dates without a leading zero.

Is it still necessary or useful to run the high_group_loader rake task, since the SSA has implemented randomized assignment?  If yes, it looks like their website started documenting dates with no leading zero in January of '07, like 1/03/07.  The single-character change in this pull request handles that case.

If no longer necessary, perhaps it would be good to update the README.

Thanks!