Replace buildout-versions with buildout.dumppickedversions.

buildout-dumpversions makes all version pins lowercase
and this means that versions:Plone, used by the download part, is not found.
Still, other things go wrong...
Fix travis on 4.3.x.

Somehow versions:Plone is not defined (and versions:plone also not),
so buildout-cache.cfg cannot get a buildout:basename and download:url.
There is some sillyness going on, even when not using any buildout extensions.