Builds including PromisesSwift failing on CI

In my iOS project I have a pod dependency which I am specifying like so:

```
pod 'PromisesSwift'
```

For some reason, the dependency `PromisesObjC` is appearing twice in the Podfile.lock, even when I delete this file and the `Pods` directory before running `pod install` and `pod update`.

```
- PromisesObjC (2.2.0)
- PromisesSwift (2.2.0):
- PromisesObjC (= 2.2.0)
```

When I try and build this project on CI I get the following issue:

```
[!] CocoaPods could not find compatible versions for pod "PromisesObjC":
In snapshot (Podfile.lock):
PromisesObjC (= 2.2.0)
In Podfile:
PromisesSwift was resolved to 2.2.0, which depends on
PromisesObjC (= 2.2.0)
None of your spec sources contain a spec satisfying the dependencies: `PromisesObjC (= 2.2.0), PromisesObjC (= 2.2.0)`.
```

How does this make any sense? Both versions are identical and yet it cannot resolve them.

I've also tried disintegrating the project and reinstalling with no luck.