release.py: Simplify the anchor calculation further.

* tools/dist/release.py
  (Version.is_recommended, Version.get_download_anchor): New functions.
  (write_announcement): Use the branch attribute of the Version object
    for major_minor, remove anchor logic that is now in Version object.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1595948 13f79535-47bb-0310-9956-ffa450edef68