travis: build against branched version of lxd

Since we're using github.com/lxc/lxd as our import, if a pull request has
modified the library lxd, without this step we use the unmodified tree at
github.com/lxc/lxd, which doesn't have the pull request's changes, so we
manually overwrite the imported version with our changes.

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>