also slightly speed up writing to the cache.
For example, on the Lyft codebase with 1,500 Swift files:
```bash
$ time swiftlint lint --quiet
swiftlint --quiet 3.53s user 0.27s system 388% cpu 0.979 total
$ rm -rf ~/Library/Caches/SwiftLint && time swiftlint lint --quiet
swiftlint --quiet 35.20s user 1.22s system 371% cpu 9.806 total
$ time swiftlint lint --quiet
swiftlint lint --quiet 0.90s user 0.13s system 218% cpu 0.472 total
$ rm -rf ~/Library/Caches/SwiftLint && time swiftlint lint --quiet
swiftlint lint --quiet 31.78s user 1.18s system 360% cpu 9.146 total
```