Update VS version info to match 2017 RC

ILSpy.sln

@@ -1,7 +1,7 @@
 
 Microsoft Visual Studio Solution File, Format Version 12.00
-# Visual Studio 14
-VisualStudioVersion = 14.0.25420.1
+# Visual Studio 15
+VisualStudioVersion = 15.0.25920.0
 MinimumVisualStudioVersion = 10.0.40219.1
 Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "doc", "doc", "{F45DB999-7E72-4000-B5AD-3A7B485A0896}"
 	ProjectSection(SolutionItems) = preProject

ILSpy/ILSpy.csproj

@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="utf-8"?>
-<Project ToolsVersion="4.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003" DefaultTargets="Build">
+<Project ToolsVersion="15.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003" DefaultTargets="Build">
   <PropertyGroup>
     <ProjectGuid>{1E85EFF9-E370-4683-83E4-8A3D063FF791}</ProjectGuid>
     <Configuration Condition=" '$(Configuration)' == '' ">Debug</Configuration>