remove the private key first if we already have a cert loaded prior to adding
it to the agent
Merge pull request #4 from synfinatic/remove-cert-b4-add

Remove private key from ssh-agent before re-adding

Works around a bug, at least in openssh on os x 10.9.2, where re-adding a private key that has a new cert puts the agent into a weird state where there are many certs loaded for a single key. In that case none of the certs work.