Updated Video.java to use String for Id.

MongoDB's auto generated ObjectId consists of alphanumeric characters and hyphens '-'. When a long is used as the datatype for the Id, Spring Data MongoDB will issue an update/overwrite a single document in MongoDB with '_Id': NumberLong(0). 

Output from mongo shell (Id as long):

> db.video.find().pretty()
{
	"_id" : NumberLong(0),
	"_class" : "org.magnum.mobilecloud.video.repository.Video",
	"name" : "Video-3a3b3158-9517-48c4-bdbd-718374e296bd",
	"url" : "http://coursera.org/some/video-3a3b3158-9517-48c4-bdbd-718374e296bd",
	"duration" : NumberLong(2880000)
}

This differs from the desired outcome which is to create a new (additional) document on subsequent calls to src / test / java / org / magnum / mobilecloud / integration / test / VideoSvcClientApiTest.java

Output from mongo shell (Id as String in 2nd document):

> db.video.find().pretty()
{
	"_id" : NumberLong(0),
	"_class" : "org.magnum.mobilecloud.video.repository.Video",
	"name" : "Video-3a3b3158-9517-48c4-bdbd-718374e296bd",
	"url" : "http://coursera.org/some/video-3a3b3158-9517-48c4-bdbd-718374e296bd",
	"duration" : NumberLong(2880000)
}
{
	"_id" : ObjectId("545cba6444ae69b880ba589b"),
	"_class" : "org.magnum.mobilecloud.video.repository.Video",
	"name" : "Video-a820232d-dca3-4754-8962-f5355e408d61",
	"url" : "http://coursera.org/some/video-a820232d-dca3-4754-8962-f5355e408d61",
	"duration" : NumberLong(60000)
}