Adds call to didMoveToParentViewController

In the main view controller when the collection view controllers
are created and then appended as sub view controller a mandatory
call to didMoveToParentViewController was missing. For more
information check https://developer.apple.com/library/ios/documentation/uikit/reference/UIViewController_Class/Reference/Reference.html#//apple_ref/doc/uid/TP40006926-CH3-SW81