Support `QuickCheck` version `2.17.1.0` (`text-iso8601`).

Version [`2.17.1.0`](https://hackage.haskell.org/package/QuickCheck-2.17.1.0) of the `QuickCheck` package has been released, but the most recent revision of `text-iso8601` on Hackage ([`0.1.1-r2`](https://hackage.haskell.org/package/text-iso8601-0.1.1/revision/2.cabal)) has an effective upper bound of `< 2.16` for the test suite:

```cabal
      QuickCheck            >=2.14.3   && <2.16
```

