Fixes #472: Remove the use of System.Diagnostics.Debug.Assert

Direct use of `System.Diagnostics.Debug.Assert()` causes issue in
production environment.  Use Unity version of [Debug.Assert()](https://docs.unity3d.com/ScriptReference/Debug.Assert.html)
instead.

source/VersionHandlerImpl/src/VersionHandlerImpl.cs

@@ -991,7 +991,7 @@ public FileMetadataByVersion(string filenameCanonical) {
         /// Add metadata to the set.
         /// </summary>
         public void Add(FileMetadata metadata) {
-            System.Diagnostics.Debug.Assert(
+            Debug.Assert(
                 filenameCanonical == null ||
                 metadata.filenameCanonical.Equals(filenameCanonical));
             metadataByVersion[metadata.CalculateVersion()] = metadata;
@@ -2311,7 +2311,7 @@ static void UpdateVersionedAssetsOnUpdate() {
         if (!Enabled) {
             return;
         }
-        
+
         UpdateVersionedAssets();
         NotifyWhenCompliationComplete(false);
         UpdateAssetsWithBuildTargets(EditorUserBuildSettings.activeBuildTarget);