[check_postgres] Improvement to eliminate critical notification for new_version_pg

[Emre Hasegeli]

 new_version_pg check exits with critical if new minor version is 
 available. For us who use check_postgres with Nagios effectively it is a 
 problem because monitoring department wake administrators up if 
 something turns to critical.

 On the other hand the behavior is logical because it exits with warning 
 if new major version is available. So I think a check to check only 
 minor versions and return warning would be nice. What do you thing? Is 
 it okay if I add this?