upgrade C# protobuf dependencies to 3.0.0-beta3

templates/src/csharp/build_packages.bat.template

@@ -4,7 +4,7 @@
   
   @rem Current package versions
   set VERSION=${settings.csharp_version}
-  set PROTOBUF_VERSION=3.0.0-beta2
+  set PROTOBUF_VERSION=3.0.0-beta3
   
   @rem Packages that depend on prerelease packages (like Google.Protobuf) need to have prerelease suffix as well.
   set VERSION_WITH_BETA=%VERSION%-beta