Fix some of the issues with duration sometimes not showing up

* duration was not actually defined in Travis.Job, which may have caused
  bugs when combined with partial record loading (so for example it was
  ok on a refresh, but may have been broken on update)
* notStarted was not a dependency of duration

assets/scripts/app/models/extensions.coffee

@@ -6,7 +6,7 @@ Travis.DurationCalculations = Ember.Mixin.create
       duration
     else
       Travis.Helpers.durationFrom(@get('startedAt'), @get('finishedAt'))
-  ).property('_duration', 'finishedAt', 'startedAt')
+  ).property('_duration', 'finishedAt', 'startedAt', 'notStarted')
 
   updateTimes: ->
     unless @get('isFinished')

assets/scripts/app/models/job.coffee

@@ -21,6 +21,7 @@ require 'travis/model'
   annotations: Ember.hasMany('Travis.Annotation')
 
   _config: Ember.attr('object', key: 'config')
+  _duration: Ember.attr(Number, key: 'duration')
 
   log: ( ->
     @set('isLogAccessed', true)