Set correct version number at Windows installer based on
Go's Mercurial tag.

Name                                     | Version
------------------------------------------------
Go Programming Language amd64 go1.3.3    | 1.3.3
Go Programming Language amd64 go1.2rc3   | 1.2
Go Programming Language amd64 go1.2beta1 | 1.2

Fixes #8239.

LGTM=adg
R=adg, c.emil.hessman, alex.brainman
CC=golang-codereviews
https://golang.org/cl/160950044