Fix for #63 (comment)

Obj2Tiles/Options.cs

@@ -47,7 +47,7 @@ public sealed class Options
     [Option("keep-intermediate", Required = false, HelpText = "Keeps the intermediate files (do not cleanup)", Default = false)]
     public bool KeepIntermediateFiles { get; set; }
 
-    [Option('t', "y-up-to-z-up", Required = true, HelpText = "Convert the upward Y-axis to the upward Z-axis, which is used in some situations where the upward axis may be the Y-axis or the Z-axis after the obj is exported.", Default = true)]
+    [Option('t', "y-up-to-z-up", Required = false, HelpText = "Convert the upward Y-axis to the upward Z-axis, which is used in some situations where the upward axis may be the Y-axis or the Z-axis after the obj is exported.", Default = true)]
     public bool? YUpToZUp { get; set; }
 }