Fix manifest verification throwing an exception for invalid version numbers.

Fixes #37.

Author: BenLubar

src/dotnet/upack/Command.cs

@@ -285,7 +285,7 @@ internal static string ValidateManifest(UniversalPackageMetadata info)
                 }
             }
 
-            if (info.Version == null)
+            if (UniversalPackageVersion.TryParse(info["version"] as string ?? string.Empty) == null)
             {
                 return "missing or invalid version.";
             }