[PATCH] Fixed bug where is_core assumed linear release sequence

If you specified a version of the module, is_core has to track through
releases, as the %delta data structure only records where a module
version number changes in core, not every module version number in every release.
I was naively trawling the releases in numerical order, but %delta includes
information that let's you construct the release tree.

This fix only traverses the branch of the overall release tree that leads
to the specified Perl release. Further explanation and example in blog post:

    http://neilb.org/2013/09/21/adding-is-core.html

Signed-off-by: Chris 'BinGOs' Williams <chris@bingosnet.co.uk>