haxelib selfupdate says to use haxe update.hxml (since 3.1 ?)

Otherwise `haxe upgrade.hxml` says it "can't find upgrade.hxml". Maybe a haxe 3.1 thing ? I don't know, I haven't used haxe for a long time.
Merge pull request #10 from julsam/patch-1

haxelib selfupdate says to use haxe update.hxml (since 3.1 ?)