Change dependency to BouncyCastle.Cryptography instead of Portable.BouncyCastle

[`BouncyCastle.Cryptography`](https://www.nuget.org/packages/BouncyCastle.Cryptography/) is the new official nuget package supporting newer .NET frameworks. The dependency on the unsupported [`Portable.BouncyCastle`](https://www.nuget.org/packages/Portable.BouncyCastle) brings unsolvable clashes when trying to add the former to a project using TimeStampClient.