Allow partially-trusted callers again.

Fixes issue #552. (And yay, it looks like our build profile supports this...)

csharp/src/Google.Protobuf/Properties/AssemblyInfo.cs

@@ -32,6 +32,7 @@
 
 using System.Reflection;
 using System.Runtime.CompilerServices;
+using System.Security;
 
 // General Information about an assembly is controlled through the following 
 // set of attributes. Change these attribute values to modify the information
@@ -45,6 +46,7 @@ using System.Runtime.CompilerServices;
 [assembly: AssemblyCopyright("Copyright ©  2015")]
 [assembly: AssemblyTrademark("")]
 [assembly: AssemblyCulture("")]
+[assembly: AllowPartiallyTrustedCallers]
 
 #if SIGNED
 [assembly: InternalsVisibleTo("Google.Protobuf.Test, PublicKey=" +