Fix minor bug in RevisionStatTimeslice. Currently REVISION_TIMEFRAME …

…is set to 7 days. If the course start is less that one week ago, we don't want to try to retrieve a timeslice for 7 days ago because that timeslice won't exist. Because of that, now we fix the start period for the timeframe as the max between course start or 7 days ago. (#6186)

lib/revision_stat_timeslice.rb

@@ -7,7 +7,7 @@ class RevisionStatTimeslice
   def initialize(course, end_period = Time.zone.now)
     @course = course
     @end_period = end_period
-    @start_period = REVISION_TIMEFRAME.days.ago
+    @start_period = [REVISION_TIMEFRAME.days.ago, course.start].max
   end
 
   def recent_revisions_for_course