KeyPair isKey assertion should be min-versioned

[arekinath]

Currently when there's an sshpk release newer than the sshpk-agent one available and there's a Key format change, you can get errors like this:

mls: AssertionError: options.public must be a sshpk.Key instance
    at AgentKeyPair.SDCKeyPair (/usr/local/lib/node_modules/manta/node_modules/smartdc-auth/lib/keypair.js:50:14)
    at new AgentKeyPair (/usr/local/lib/node_modules/manta/node_modules/smartdc-auth/lib/kr-agent.js:93:10)
    at /usr/local/lib/node_modules/manta/node_modules/smartdc-auth/lib/kr-agent.js:81:14
    at Request.r_cb (/usr/local/lib/node_modules/manta/node_modules/sshpk-agent/lib/client.js:59:3)
    at Request.state_done (/usr/local/lib/node_modules/manta/node_modules/sshpk-agent/lib/client-fsm.js:224:8)
    at Request.FSM._gotoState (/usr/local/lib/node_modules/manta/node_modules/mooremachine/lib/fsm.js:325:4)
    at FSMStateHandle.gotoState (/usr/local/lib/node_modules/manta/node_modules/mooremachine/lib/fsm.js:76:23)
at Client.<anonymous> (/usr/local/lib/node_modules/manta/node_modules/sshpk-agent/lib/client-fsm.js:190:5)
    at emitOne (events.js:96:13)
    at Client.emit (events.js:188:7)

This is because in keypair.js we're using Key.isKey from sshpk in an assertion without giving it a minimum version we require, so it's assuming we need the latest.