Podspec swift version must be updated.

Hi Dears,
The framework's swift version has been updated to 5.0. however, the swift version in pospec needs to be updated to the latest swift version.

https://github.com/airbnb/MagazineLayout/blob/d2eb5c5eb04cb9247c1e1446cf63a89ff513b0b9/MagazineLayout.podspec#L9

the `pod install` might face the error where `Podfile` has specified swift version support.
`supports_swift_versions '>= 5.0'`

```bash
[!] Unable to determine Swift version for the following pods:

- `MagazineLayout` does not specify a Swift version (`4.0`) that is satisfied by any of targets integrating it.
```

please update the swift version in pod spec file.

Open MagazineLayout.podspec at line 9 and compare its declared Swift version with the framework's Swift 5.0 requirement. Run pod install with a Podfile using supports_swift_versions '>= 5.0'; done when CocoaPods accepts MagazineLayout without the shown version error.

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