Remove 'latest'LangVersion added by Unity 2018.3 which is not supported by VS 2015

EditorScript.cs

using System.IO;
using System.Linq;
using System.Text;
using System.Xml.Linq;

using UnityEditor;

#if ENABLE_VSTU

using SyntaxTree.VisualStudio.Unity.Bridge;

[InitializeOnLoad]
public class ProjectFileHook
{
	static ProjectFileHook()
	{
		ProjectFilesGenerator.ProjectFileGeneration += (string name, string content) =>
		{
			return content.Replace("<LangVersion>latest</LangVersion>", string.Empty);
		};
	}
}

#endif