Make zinc relative its -classpath argument too.  (Again, because mesos
creates long-ass paths).

Testing Done:
Ran new tests.  Also https://travis-ci.org/pantsbuild/pants/builds/44629688

Reviewed at https://rbcommons.com/s/twitter/r/1528/
Sync of https://github.com/pantsbuild/pants/commit/8906cd9d0a858ce93a5bdd70a7aca0233ba6612f