Prefer the newest version of Lua before specifically looking for version 5.1

This fixes building on Fedora > 19 which come with lua-5.2 (but still works with Fedora 19 which comes with lua-5.1)

Here's an example of a failing build without this patch: https://kojipkgs.fedoraproject.org//work/tasks/5559/9415559/build.log