Added flag --pod-versions to list command

This will add the flag --pod-versions to the list command so you can do the following to see the latest versions of all pods:

> pod list --pod-versions

  500px-iOS-api 1.0.5
  _.m 0.1.1
  A2DynamicDelegate 2.0.2
  A2StoryboardSegueContext 1.0.1
  A3GridTableView 0.0.1
  AAActivityAction 1.0.1
  AAImageUtils 0.1.0
  AALaunchTransition 0.1.3
  ABCalendarPicker 1.1.2
  ABContactHelper 0.0.2
  ABCustomUINavigationController 1.0
  ABGetMe 1.0.0
  ABMultiton 1.2.1
  ABRequestManager 0.0.1
  ABTest 0.0.5
  AccordionView 0.0.2
  ACEAutocompleteBar 1.0.0
  ACEDrawingView 1.1.0
  ACEExpandableTextCell 0.0.1
  ACEToolKit 0.0.3
  ACEView 0.0.4