make DistributionUpdate download build-version files and use them in the info files

Also automated downloading the builds, finally. I had done it by hand until
now.

Note that the Windows autobuilder has an expired cert, so it will refuse to
download from it currently. I have emailed its admin to get that fixed,
hopefully.

This commit was sponsored by Peter Hogg.

Build/Version.hs

@@ -14,6 +14,8 @@ import Build.TestConfig
 import Utility.Monad
 import Utility.Exception
 
+type Version = String
+
 {- Set when making an official release. (Distribution vendors should set
  - this too.) -}
 isReleaseBuild :: IO Bool
@@ -25,7 +27,7 @@ isReleaseBuild = isJust <$> catchMaybeIO (getEnv "RELEASE_BUILD")
  -
  - If git or a git repo is not available, or something goes wrong,
  - or this is a release build, just use the version from the changelog. -}
-getVersion :: IO String
+getVersion :: IO Version
 getVersion = do
 	changelogversion <- getChangelogVersion
 	ifM (isReleaseBuild)
@@ -41,7 +43,7 @@ getVersion = do
 				else return $ concat [ major, ".", autoversion ]
 		)
 	
-getChangelogVersion :: IO String
+getChangelogVersion :: IO Version
 getChangelogVersion = do
 	changelog <- readFile "debian/changelog"
 	let verline = takeWhile (/= '\n') changelog