Supply "5.x-thr" versions.

[schwern]

Currently if one wants to test Perl with threads, one has to know the specific threaded Perl versions available from https://github.com/travis-perl/builder/blob/master/.travis.yml This isn't well documented and it's fussy. I naively used 5.10-thr in my projects and only just noticed it is building against some other perl.

It would be much easier if a shorthand version existed to use the newest threaded version in that series. So if 5.10 uses 5.10.1 then 5.10-thr would use 5.10.1-thr.