Downgrade to Roslyn 2.4 in AddIn to support earlier versions of VS 2017.

ILSpy.AddIn/ILSpy.AddIn.csproj

@@ -46,10 +46,10 @@
   <ItemGroup>
     <PackageReference Include="EnvDTE" Version="8.0.2" />
     <PackageReference Include="EnvDTE80" Version="8.0.2" />
-    <PackageReference Include="Microsoft.CodeAnalysis" Version="2.7.0" />
-    <PackageReference Include="Microsoft.CodeAnalysis.CSharp" Version="2.7.0" />
+    <PackageReference Include="Microsoft.CodeAnalysis" Version="2.4.0" />
+    <PackageReference Include="Microsoft.CodeAnalysis.CSharp" Version="2.4.0" />
     <PackageReference Include="Microsoft.VisualStudio.ComponentModelHost" Version="15.6.27413" />
-    <PackageReference Include="Microsoft.VisualStudio.LanguageServices" Version="2.7.0" />
+    <PackageReference Include="Microsoft.VisualStudio.LanguageServices" Version="2.4.0" />
     <PackageReference Include="Microsoft.VisualStudio.Shell.14.0" Version="14.3.25407" />
     <PackageReference Include="Mono.Cecil" Version="0.10.0-beta7" />
     <PackageReference Include="VSLangProj" Version="7.0.3301" />