windows versions on download page

[Jack Ostroff]

The download page has two links for windows versions.   The link for  
"download server" has links for 5.0.6 from last September.  The link  
for "preview builds" is for nightly builds from master branch.  Should  
we add the link to the Jenkins page of nightly builds from the 5.1  
branch, and should the download server get a more recent version?

Jack