Now in addition to enrollment_start support in ExperimentWaffleFlag,
you can set an enrollment_end date for your experiment. All enrollments
after this date will get the control experience.
If the user is not logged in, the ExperimentWaffleFlag code was
raising an exception when trying to send an event to segment.
This is a quick fix to ignore anonymous users.
This is a helpful class when running an experiment, to help
bucket users and keep track of which enrollments to consider
as part of the experiment.
AA-53
