perl 5.8 is actually supported on travis-ci now

I ran travis-lint on my .travis.yml file and found it complained
about perl 5.8. Tests for 5.8 are however executing just fine:

https://travis-ci.org/mbeijen/DBD-mysql-1/builds/12478198

See also http://about.travis-ci.org/docs/user/ci-environment/#Perl-VM-images

This commit makes sure specifying 5.8 as a perl version is allowed.