Update no_shelllogin_for_systemaccounts.xml

Regex matches gid < 500, instead of uid < 500 as it should.  

Changed the order of the uid and gid match regexes chunks so that it matches the accounts with uid < 500 instead of the current match of gid < 500.
Merge pull request #354 from rrenshaw/sysaccount_user_lt_500

Update no_shelllogin_for_systemaccounts.xml