Always relativize the classpath where possible.

Mesos produces very long path names, and repeating these names in the classpath can exceed the max length of a single argument (which defaults to 131072 on Linux).

Testing Done:
https://travis-ci.org/pantsbuild/pants/builds/43541955

(and then ran new tests locally).

Reviewed at https://rbcommons.com/s/twitter/r/1455/