Update to System.IdentityModel.Tokens.Jwt v4.0.0

Hello,

I have an MVC4 app that uses the latest version (v4.0.0) of System.IdentityModel.Tokens.Jwt. I'd like to use the CORS pieces of Thinktecture.IdentityModel.45 in my app, but I can't seem to get that going unless both are on the latest System.IdentityModel.Tokens.Jwt package.

This PR updates to the latest version.