Revert "bump(fleet): Upgrade to v0.5.1"

Rolling back to 0.5.0 due to a regression in the etcd client.

Issue: https://github.com/coreos/fleet/issues/582

This (mostly) reverts commit 6a01e3c1e58c2f551307913f75097e09e73d539c.
Merge pull request #703 from marineam/fleet

Revert "bump(fleet): Upgrade to v0.5.1"