Fixed lp:1614559: Check LXD API is 1.X, not LXD server binary version

[dimitern]

We were checking the LXD server's binary version was 2.0.0, which is not
the right thing to check for compatibility. This PR updates the lxc/lxd
dependencies to the latest version, so we can use server.APIVersion
field and actually perform the correct check - whether the LXD API
version is 1.X at least. When it isn't Juju we still try to use it, but clearly
warn the user about it being unsupported. This is to allow future LXD
API versions to work with older Juju releases, assuming the API itself
does not break backwards-compatibility.

See bug http://pad.lv/1614559 for details.

QA steps:
1. sudo add-apt-repository ppa:ubuntu-lxc/lxd-stable
2. sudo apt update && sudo apt full-upgrade -y
3. lxc version # ensure it reports 2.1
4. cd $GOPATH/src/github.com/juju/juju/
5. git checkout origin/master
6. juju bootstrap lxd localhost # expect failure
7. git checkout dimitern/lp-1614559-lxd-api-version
8. godeps -u dependencies.tsv
9. go install -v github.com/juju/juju/...
10. juju bootstrap lxd localhost # expect success

(Review request: http://reviews.vapour.ws/r/5496/)

[dimitern]

$$fixes-1614559$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju