Use TimeHelper in rake task

lib/tasks/detect_exercise_anomalies.rake

@@ -22,6 +22,8 @@ namespace :detect_exercise_anomalies do
   AVERAGE_WORKING_TIME_CACHE = {}
 
   task :with_at_least, [:number_of_exercises, :number_of_solutions] => :environment do |task, args|
+    include TimeHelper
+
     number_of_exercises = args[:number_of_exercises]
     number_of_solutions = args[:number_of_solutions]
 
@@ -71,14 +73,6 @@ namespace :detect_exercise_anomalies do
     end
   end
 
-  def time_to_f(timestamp)
-    unless timestamp.nil?
-      timestamp = timestamp.split(':')
-      return timestamp[0].to_i * 60 * 60 + timestamp[1].to_i * 60 + timestamp[2].to_f
-    end
-    nil
-  end
-
   def get_average_working_time(exercise)
     unless AVERAGE_WORKING_TIME_CACHE.key?(exercise.id)
       seconds = time_to_f exercise.average_working_time