Troubleshoot Travis config

- Try removing .net core from mono build environment

.travis.yml

@@ -1,7 +1,6 @@
 language: csharp
 solution: MatterControl.sln
 mono: latest
-dotnet: 2.0.0
 script:
   - nuget restore MatterControl.sln
   - msbuild MatterControl.sln

MatterControlLib/DesignTools/Interfaces/IPropertyGridModifier.cs

@@ -1,5 +1,5 @@
 /*
-Copyright (c) 2018, Lars Brubaker, John Lewin
+Copyright (c) 2019, Lars Brubaker, John Lewin
 All rights reserved.
 
 Redistribution and use in source and binary forms, with or without
@@ -27,11 +27,10 @@ of the authors and should not be interpreted as representing official policies,
 either expressed or implied, of the FreeBSD Project.
 */
 
-using MatterHackers.Agg.UI;
-using MatterHackers.DataConverters3D;
-using MatterHackers.MatterControl.PartPreviewWindow;
 using System;
 using System.Collections.Generic;
+using MatterHackers.Agg.UI;
+using MatterHackers.DataConverters3D;
 
 namespace MatterHackers.MatterControl.DesignTools
 {