chore(confd): bump to v0.5.0-json

This is a hand-compiled confd v0.5.0 with a patch to add JSON functions.
Since it's just adding new functions, it should be completely safe for
master. And I'll need this soon, so it'd be good to build base at the
Docker Hub with this change.
Merge pull request #12 from deis/confd-v0.5.0-json

chore(confd): bump to v0.5.0-json