Relax stdlib version dependency to >= 3.0

The strict dependency on 4.x meant that the module could not co-exist with
any other modules that already depend on 3.x

Strictly we don't need 4.x - it would just be nice to have the ability to
refresh anchor patterns. stdlib 3.x means that we can still support Puppet
3.x, but also Puppet 2.7 if necessary.