Add mapping for REASON_TASK_KILLED_DURING_LAUNCH

[dposada]

What?

Add a mapping for REASON_TASK_KILLED_DURING_LAUNCH:

http://mesos.apache.org/documentation/latest/task-state-reasons/#for-state-task_killed

Similar to past issue #28.

Why?

This reason comes through quite frequently in the wild, so it would be great to have first-class support for it. As it stands right now, we're having to "roll our own" to handle this reason cleanly.