Stop supporting Leopard

Is this really Misty?!

Library/brew.rb

@@ -43,9 +43,9 @@ case HOMEBREW_PREFIX.to_s when '/', '/usr'
   # it may work, but I only see pain this route and don't want to support it
   abort "Cowardly refusing to continue at this prefix: #{HOMEBREW_PREFIX}"
 end
-if OS.mac? and MacOS.version < "10.5"
+if OS.mac? and MacOS.version < "10.6"
   abort <<-EOABORT.undent
-    Homebrew requires Leopard or higher. For Tiger support, see:
+    Homebrew requires Snow Leopard or higher. For Tiger and Leopard support, see:
     https://github.com/mistydemeo/tigerbrew
   EOABORT
 end