Fix Exporter non-upgradeable on perls prior to 5.8

The original breakage on CPAN introduced here:
https://metacpan.org/diff/release/FERREIRA/Exporter-5.63/TODDR/Exporter-5.64_04#Makefile.PL

According to Reini this is to "freeze" the Exporter version for 5.6 users (very weird claim but whatever). As a result of the above change in 5.64_04 `Perl Makefile.PL && make && make test && make install` appears to work 5.6.X but does not actually upgrade Exporter because this perl's @INC will never run across the new Exporter.pm

If this is unacceptable (i.e. Exporter must remain frozen on 5.6 - lame but possibly justifiable) - please release Exporter with perl min-ver reqiuired of 5.8.0, so at least the breakage is apparent and not silent.