Fixes a MassAssignmentSecurity error, Rails 3.2.12

When calling 
    track! MyMetric

I was getting a 500 under Rails 3.2.12, vanity 1.8.

Completed 500 Internal Server Error in 5ms

ActiveModel::MassAssignmentSecurity::Error (Can't mass-assign protected attributes: date, index, value)

I don't get the 500 any more, after this patch.