version: report peer version using --version

By calling the peer's script with --version, we can make `slc --version`
report the version as the peer chooses, instead of only the package
version, like:

    % slc -v
    strong-cli v2.5.3 (node v0.10.26)
    node-inspector v0.8.0
    strong-cluster-control v0.4.0
    strong-registry v1.1.0
    strong-supervisor v0.2.2 (strong-agent v0.4.8, strong-cluster-control v0.4.0)

Prior to this, strong-agent and strong-cluster-control versions would be
unknown.