Set C# language version to 5 for release and debug configuration.

src/Renci.SshNet/Renci.SshNet.csproj

@@ -36,6 +36,7 @@
     <NoWarn>
     </NoWarn>
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
+    <LangVersion>5</LangVersion>
   </PropertyGroup>
   <PropertyGroup>
     <SignAssembly>true</SignAssembly>