For those already using Let's Encrypt since the closed beta: Do not forget to remove the `agree-dev-preview` flags, as newer client version do seem to throw up

For those already using Let's Encrypt since the closed beta: Do not forget to remove the `agree-dev-preview` flags, as newer client version do seem to throw up if it's still set. I had `agree-dev-preview = True` in a config file and got an error about True being an invalid value.