Fix Unity version conditional

If 2018.3 is allowed (as the message implies), conditional should be UNITY_2018_3_OR_NEWER, not UNITY_2018_4_OR_NEWER

Author: michael-house

Assets/MixedRealityToolkit/Utilities/Editor/EditorProjectUtilities.cs

@@ -29,7 +29,7 @@ static EditorProjectUtilities()
         /// </remarks>
         public static void CheckMinimumEditorVersion()
         {
-#if !UNITY_2018_4_OR_NEWER
+#if !UNITY_2018_3_OR_NEWER
             DisplayIncorrectEditorVersionDialog();
 #endif
         }