Add <LangVersion>6</LangVersion> to the core protobuf C# project to prevent future incompatibility

csharp/src/Google.Protobuf/Google.Protobuf.csproj

@@ -5,6 +5,7 @@
     <Copyright>Copyright 2015, Google Inc.</Copyright>
     <AssemblyTitle>Google Protocol Buffers</AssemblyTitle>
     <VersionPrefix>3.6.1</VersionPrefix>
+    <LangVersion>6</LangVersion>
     <Authors>Google Inc.</Authors>
     <TargetFrameworks>netstandard1.0;net45</TargetFrameworks>
     <GenerateDocumentationFile>true</GenerateDocumentationFile>