Need replace Timex.DateTime.now -> Timex.now in Readme

[Salisbury]

perform_at = Timex.shift(Timex.DateTime.now, seconds: 30)

raise error:

(UndefinedFunctionError) function Timex.DateTime.now/0 is undefined (module Timex.DateTime is not available)

Should use Timex.now

(see https://github.com/bitwalker/timex)