dust off the build process: start by pinning magic-wormhole-0.4.0
dust off the build process: start by pinning magic-wormhole-0.4.0

This was the last version of magic-wormhole that provided the API which
petmail currently uses. Magic-wormhole-0.4.0 doesn't actually work
anymore (the server protocol has since been changed, and the old server shut
down), but this is enough to let "setup.py safe_develop; petmail test" mostly
work.

warner/petmail,warner/petmail,warner/petmail