1.20.x client cannot communicate with 1.22.x env

[Curtis Hovey]

A 1.20.x client cannot talk to an env bootstrapped with 1.22.x. The client sees this error

2015-05-13 19:42:28 DEBUG juju.state.api apiclient.go:248 error dialing "wss://8.19.33.120:17070/environment/ac6e0cab-9f6c-4e33-85c0-bac5402d4e62/api", will retry: websocket.Dial wss://8.19.33.120:17070/environment/ac6e0cab-9f6c-4e33-85c0-bac5402d4e62/api: x509: certificate is valid for localhost, juju-apiserver, juju-mongodb, not anything

1.20.x clients can talk to envs upgraded from 1.20 such as going to 1.21, 1.22, and finally 1.23.

To reproduce.
Install 1.22 (1.22.1 is in trusty proposed)
juju bootstrap
juju status
install 1.20 (1.20.11 is in trusty updates)
juju --debug status
...

Note that this path is fine.
install 1.20 (1.20.11 is in trusty updates)
juju bootstrap
juju status
juju upgrade --version=1.21.1
juju status
juju upgrade --version=1.22.3
juju status
juju upgrade --version=1.23.2
juju status