Differentiate sortOrder for repos without lastBuildFinishedAt

All of the repos without lastBuildFinishedAt had the same value for
sortOrder, which could result in 'jumps' on the list when loading new
data from the API.

assets/javascripts/app/models/repository.coffee

@@ -42,7 +42,11 @@ require 'travis/model'
   ).property('data.last_build_duration', 'lastBuildStartedAt', 'lastBuildFinishedAt')
 
   sortOrder: (->
-    - new Date(@get('lastBuildFinishedAt') || '9999').getTime() # cuz sortAscending seems buggy when set to false
+    # cuz sortAscending seems buggy when set to false
+    if lastBuildFinishedAt = @get('lastBuildFinishedAt')
+      - new Date(lastBuildFinishedAt).getTime()
+    else
+      - new Date('9999').getTime() + parseInt(@get('id'))
   ).property('lastBuildFinishedAt')
 
   stats: (->