Force Kryo version 2.24.0

This resolves a dependency convergence error.  titan-core depended
directly on 2.23.0, but gremlin-core depended on 2.24.0.  This commit
opts for the newer of the two.

+-com.thinkaurelius.titan:titan-core:0.9.0-SNAPSHOT
  +-com.tinkerpop:gremlin-core:3.0.0-SNAPSHOT
    +-com.esotericsoftware.kryo:kryo:2.24.0