juju 1.19.4

Update devel to juju devel 1.19.4. Juju moved to github during this release, changing the location of their files in the source tree. When stable 1.20.0 is released in a couple weeks, I will remove support for launchpad.net/juju-core files.
Use `build.stable?` or `build.devel?` instead of sniffing the url.