🍫 Podspec: Unrecognized `swift_version` key.

The error below does not appear when running `pod lib lint` but it does appear while attempting to push a new spec version to trunk.

```
$ pod trunk push Commandant.podspec
...
[!] The Pod Specification did not pass validation.
The following validation failed:
- Warnings: Unrecognized `swift_version` key.
```

This is using CocoaPods 1.6.1.