See https://github.com/joeyh/git-annex/pull/55.

### Please describe the problem.
Solver needs some help when building with +S3 and +Webapp for Homebrew.

### What steps will reproduce the problem?
Build git-annex with GHC 8.0.1 and cabal install 1.24.0.0 with --flags=s3\ webapp

### What version of git-annex are you using? On what operating system?
6.20160719
macOS 10.11.5

### Please provide any additional information below.
git-annex.cabal needs a couple of tweaks to reflect the following known issues:


1.
aws 0.14.0 is incompatible with http-conduit 2.2.0
https://github.com/aristidb/aws/issues/206

2.
esqueleto 2.4.3 is incompatible with persistent 2.5
https://github.com/prowdsponsor/esqueleto/issues/137
https://github.com/prowdsponsor/esqueleto/pull/141
https://github.com/prowdsponsor/esqueleto/pull/139

> I assume this will be sorted out upstream, but I don't mind temporarily
> adding the constraints. [[done]] --[[Joey]]
