Fix the build problem by updating ssh package
Merge branch 'update-ssh-package' of github.com:higebu/packer into higebu-update-ssh-package

Conflicts:
	builder/amazon/common/ssh.go
	builder/digitalocean/ssh.go
	builder/googlecompute/ssh.go
	builder/openstack/ssh.go
	communicator/ssh/communicator_test.go
	communicator/ssh/keychain.go
	communicator/ssh/keychain_test.go