travis: build against 5.8 and 5.18

Travis now supports perls 5.8 and 5.18 natively, so let's take advantage of
that :)

Unfortunately, we cannot build against 5.8 at the moment, but 5.8 is
supported.  So... hm.  We'll need a cleverer solution here, but until then
we can mark the 5.8 build as allowed to fail.