### Please describe the problem.
Cf. https://github.com/joeyh/git-annex/pull/56
patch url: https://github.com/joeyh/git-annex/pull/56.patch

Goes together with Homebrew PR https://github.com/Homebrew/homebrew-core/pull/3724

Homebrew Jenkins CI timed out after two hours with Solver
still trying to find a solution.

### What steps will reproduce the problem?
Using --flags=s3\ webapp and --max-backjumps=10000

### What version of git-annex are you using? On what operating system?
6.20160808 macOS 10.9, 10.10, 10.11

### Please provide any additional information below.
This is a follow-up to https://github.com/joeyh/git-annex/commit/18e458db,
since there's been a regression in the situation between 6.20160619 and
6.20160808, probably simply because Hackage is a moving target.

Proposed solution is to set persistent (==2.2.4.1) instead of
persistent (<2.5) in git-annex.cabal

> [[done]]
