Allow not to retry some exceptions

[fatkodima]

This is very useful when you want to retry a lot of types of exceptions, except for, let's say, one.
And to not list all of them, you can just type :on with :not.

class UnreliableServiceException < StandardError; end
class ServiceUnavailableException < UnreliableServiceException; end
class LimitExceededException < UnreliableServiceException; end
# ... 10 more exception classes...
class AccountExpiredException < UnreliableServiceException; end

Retriable.retriable(on: UnreliableServiceException, not: AccountExpiredException) do
  # do something
end

Non whitespace changes

[Sylor-huang]

I think this is a good idea!