### Please describe the problem.
git-annex-6.20170520 no longer builds successfully. My last successful build of git-annex-6.20170520 was Jun 12, 2017, but something has probably changed in at least one of the dependencies on Hackage since then.

The build fails on all three Homebrew CI nodes (macOS 10.10, 10.11, and 10.12).

A full log is here: <https://jenkins.brew.sh/job/Homebrew%20Core%20Pull%20Requests/3175/version=sierra/consoleText>

Duplicate since that will eventually be deleted: <https://gist.github.com/ilovezfs/e93d135243d03567444eb10be8902a95>

### What steps will reproduce the problem?
Attempt to build git-annex in a cabal sandbox using cabal install.


### What version of git-annex are you using? On what operating system?
git-annex-6.20170520

### Please provide any additional information below.

The build error is

[[!format sh """
[  8 of 559] Compiling Utility.QuickCheck ( Utility/QuickCheck.hs, dist/dist-sandbox-758c2984/build/git-annex/git-annex-tmp/Utility/QuickCheck.o )

Utility/QuickCheck.hs:38:10: error:
    Duplicate instance declarations:
      instance Arbitrary EpochTime
        -- Defined at Utility/QuickCheck.hs:38:10
      instance [safe] Arbitrary Foreign.C.Types.CTime
        -- Defined in ‘Test.QuickCheck.Arbitrary’
cabal: Leaving directory '.'
cabal: Error: some packages failed to install:
git-annex-6.20170520 failed during the building phase. The exception was:
ExitFailure 1
"""]]

### Have you had any luck using git-annex before? (Sometimes we get tired of reading bug reports all day and a lil' positive end note does wonders)

Yes, thanks!

> Fixed in [[!commit 75cecbbe3fdafdb6652e95ac17cd755c28e67f20]] [[done]]
> --[[Joey]]