Make Horde.Supervisor child_spec overridable

[ordepdev]

Fixes #107

[ordepdev]

Hmm, running the tests locally it passes with:

Finished in 37.9 seconds
3 properties, 70 tests, 0 failures, 1 skipped

Still, is failing in circleci with:

1) test .set_members/2 can join and unjoin registry with set_members (ClusterTest)
     test/cluster_test.exs:164
     match (=) failed
     code:  assert 1 = Enum.count(members)
     right: 2
     stacktrace:
       test/cluster_test.exs:190: (test)

Finished in 37.6 seconds
3 properties, 70 tests, 1 failure, 1 skipped

[ordepdev]

Oh, it was my mistake 😅

[derekkraan]

Also a good chance it was a wonky test! There are a couple of those here.

[derekkraan]

This assertion isn't asserting anything I don't think. Should be ^spec I think.

[ordepdev]

Ohhh, it was testing... nothing 🙈 Already fixed!

[derekkraan]

Looks great, thanks!!