Release Icepeak version v0.5.1
Release Icepeak version v0.5.1

Changelog:

- Upgrade from stack lts-11.20 to 12.6 (this upgrades GHC from 8.2 to
  8.4)
- Upgrade "prometheus-client" from 0.3.0 to 1.0.0 (the internal API
  changed, but there are no external changes in Icepeak itself)

channable/icepeak,channable/icepeak,channable/icepeak