"stack install hakyll" on nightly - "stack invariant violated"

### General summary/comments (optional)

Attempting to install Hakyll with `--resolver nightly` gives a `stack invariant violated` error.

### Steps to reproduce

```
<*> stack install hakyll --resolver nightly
Selected resolver: nightly-2017-07-31

Error: While constructing the build plan, the following exceptions were encountered:

In the dependencies for hakyll-4.9.8.0:
pandoc must match >=1.14 && <1.20, but the stack configuration has no specified version
(latest applicable is 1.19.2.1)
pandoc-citeproc must match >=0.10.5 && <0.11, but the stack configuration has no specified version
(latest applicable is 0.10.5.1)
process-1.6.1.0 must match >=1.0 && <1.6 (latest applicable is 1.5.0.0)
time-1.8.0.2 must match >=1.4 && <1.8 (latest applicable is 1.7.0.1)
needed for unknown reason - stack invariant violated.

Recommended action: try adding the following to your extra-deps in /home/mrkgnao/.stack/global-project/stack.yaml:
- pandoc-1.19.2.1
- pandoc-citeproc-0.10.5.1
```

### Expected

I don't think `needed for unknown reason` is normal behavior -- I've never seen it before.

### Stack version

```
$ stack --version
Version 1.5.0 x86_64
```

### Method of installation

Arch Linux's `stack` package.

Start by reproducing `stack install hakyll --resolver nightly` with Stack 1.5.0 and resolver `nightly-2017-07-31`, using the dependency versions shown in the report. Trace how the build plan handles Hakyll's `pandoc`, `pandoc-citeproc`, `process`, and `time` constraints and the `needed for unknown reason` message. Done means the invariant failure is replaced by correct dependency handling or a useful diagnostic, with regression coverage.

Written by the indexing model from the issue text.