proto: make mandatory field ServiceDefinition.version optional with defa...

Hi,

I have looked at marathon.proto since version 0.1
0.1 -> 0.5
0.2 -> 0.5
0.3 -> 0.5
0.4 -> 0.5
It looks if almost all changes were compatible additions (either optional or repeated fields or complete new messages). The only incompatible change I saw was introduced in 0.5 with a required version field.
Made this field optional with default value.

Test:
Checked out tags 0.1.0, 0.2.1, 0.3.0, 0.4.0 
1) build and start each version of marathon on an empty zookeeper node /marathon-{version}
2) create 2 tasks (execute /bin/sleep)
3) stop
4) restart with current snapshot version -> make sure the tasks can be restored.

With 0.1.0 I was not able to create a task, since there was a javascript error (require.js)
With all other versions I could successfully load the tasks with 0.5.0-SNAPSHOT.
This is a rather simple test. It would be useful to have real world data.

Cheers,
Matthias

Fixes #182