vcftools: use debian-style versioning.
vcftools: use debian-style versioning.

x.xxa is incorrectly recognized as an alpha release, and x.xxa_1 is
perpetually seen as outdated.

goxberry/homebrew-science,heuermh/homebrew-science,ge11232002/homebrew-science,pditommaso/homebrew-science,maplesond/homebrew-science,DabeDotCom/homebrew-science,quantumsteve/homebrew-science,parcollet/homebrew-science,ahundt/homebrew-science,pmelsted/homebrew-science,andrewjpage/homebrew-science,kozo2/homebrew-science,dpo/homebrew-science,philippbayer/homebrew-science,joshbode/homebrew-science,aeantipov/homebrew-science