Task health checking by app version.

- HC manager now tracks checks by appId + version
- Added TaskTracker.getVersion
- Call healthCheckManager.addAllFor from
  DeploymentActor during start and restart to
  avoid a race between health check reconciliation
  and the initial task health update from the
  executor health check system
- Added missing health check reconciliation calls
- Fixes #688
- Fixes #697
fixed bug in the MarathonScheduler that led to launched tasks not being removed from the queue
added a test for readding of unlaunched tasks
MarathonScheduler: split expr to induce type check
Merge pull request #766 from mesosphere/wip-fix-task-launching-drexin

fixed bug in the MarathonScheduler that led to launched tasks not being ...
Addressed review comments for #752.

- Added internal readers-writers lock in
  MarathonHealthCheckManager.
- Cleaned up unnecessary pattern matching.
- Removed unused constructor param of type
  AppRepository in MarathonHealchCheckManager.
- Other minor refactoring.
Merge pull request #752 from mesosphere/697-health-checks-by-app-version

Task health checking by app version.
Lock handling fix in MarathonHealthCheckManager.
Added Daemon -- companies using Marathon
Merge pull request #768 from farukd/patch-1

Added Daemon -- companies using Marathon
Added DELETE /v2/deployments/{id}?force=true to
cancel a deployment without creating a new one
that rolls back to the previous state.

- Adds MarathonSchedulerService.cancelDeployment
- Fixes #755
Added docs for `DELETE /v2/deployments{id}?force`
Merge pull request #767 from mesosphere/755-delete-deployment-no-rollback

Added DELETE /v2/deployments/{id}?force=true
Setting version to 0.7.4
Setting version to 0.7.5-SNAPSHOT