Fix build when NET4_0 symbol is defined

The build was broken when the custom `NET4_0` symbol was defined:

* there was a typo in a variable name
* a couple of required using statements were missing

This problem seems to have been here from [the beginning](https://github.com/logentries/le_dotnet/commit/6017a8be0a3538e9e1dfdeda543b457aaaf40a52) so I'm not sure if the `NET4_0` symbol is actually used/needed. Without it defined the support for [`CloudConfigurationManager`](http://msdn.microsoft.com/en-us/library/microsoft.windowsazure.cloudconfigurationmanager.aspx) wouldn't work, so there's a good chance this is the root of #1