fixed reference to Json.NET in nuspec
fixed reference to Json.NET in nuspec

Json.NET package is now at version 4.5 and this is reflected in the
Cassette.nuspec file

damiensawyer/cassette,andrewdavey/cassette,BluewireTechnologies/cassette,andrewdavey/cassette,honestegg/cassette,damiensawyer/cassette,damiensawyer/cassette,andrewdavey/cassette,honestegg/cassette,BluewireTechnologies/cassette,honestegg/cassette