Change log for the probability package

0.2.6

• instance Monad Distribution.T: Remove definition of fail. This turns calls to fail into errors for GHCs prior to the "Monad Fail Proposal". Formerly it was an empty list, but this was bad since the probabilities in an empty list sum up to zero not one, thus breaking the invariant. Beginning with GHC-8.8 and the "Monad Fail Proposal" you can no longer accidentally call fail, since Distribution.T is not an instance of MonadFail.

• instance Monad Probability.EitherT: Define MonadFail instance for GHC>=8.8.1.