Serialization tests for AppUpdate
Adds disk and env to AppUpdate.

- Disallows parallel test execution
- Makes ContainerInfo a case class
- Fixes #423

Conflicts:
	project/build.scala
	src/main/scala/mesosphere/marathon/api/v2/AppUpdate.scala
	src/main/scala/mesosphere/marathon/api/v2/json/MarathonModule.scala
Backported AppUpdate serialization tests
Temporarily depend upon 0.20.0-SNAPSHOT
Merge pull request #425 from mesosphere/issue-423-AppUpdate-completeness-0.7.0-preview

Issue 423 app update completeness 0.7.0 preview