Relax monad-time bounds, and add a workaround for `now`

[414owen]

monad-time 0.4.0.0 removed an instance Monad m => MonadTime (ReaderT UTCTime m)

Which was required for now in Libjwt.Payload.

Luckily, it seems easy enough to create a NumericDate in a MonadReader UTCTime m