Update databases/redis to 4.0.6.

- This release fixes yet more errors present in the 4.0.5 fixes, that
  could affect slaves. Moreover another critical issue in quicklists,
  when they are used at a massive memory scale, was fixed in this
  release. Upgrading from any 4.0.x release, especially if you are
  running 4.0.4 or 4.0.5, is highly recommended.

(fhajny)