update ami id to fix errors

[mvasilenko]

Hello,

calling docker-machine with specified ami leads to error:

Error checking TLS connection: Error checking and/or regenerating the certs: There was an error validating certificates for host "54.183.238.197:2376": read tcp 192.168.1.92:52662->54.183.238.197:2376: read: connection reset by peer
You can attempt to regenerate them using 'docker-machine regenerate-certs [name]'.
Be advised that this will trigger a Docker daemon restart which might stop running containers.

This was a bug in docker/machine#4156 which is fixed now, I've updated ami id to recent ubuntu.

[xiaopeng163]

Thanks a lot.