Require latest C# version in Providers project

MediaBrowser.Providers/MediaBrowser.Providers.csproj

@@ -24,4 +24,8 @@
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
   </PropertyGroup>
 
+  <PropertyGroup>
+    <!-- We need at least C# 7.1 -->
+    <LangVersion>latest</LangVersion>
+  </PropertyGroup>
 </Project>