Pin the Swift version in the podspec. (#13)
Pin the Swift version in the podspec. (#13)

Part of https://github.com/material-components/material-components-ios/issues/9363

Precursor to https://github.com/material-foundation/material-testing-ios/pull/12 because pod lib lint fails otherwise:

```
 -> MDFTesting (1.0.2)
    - WARN  | [MDFTesting/MDFTesting] swift: The validator used Swift `4.0` by default because no Swift version was specified. To specify a Swift version during validation, add the `swift_versions` attribute in your podspec. Note that usage of a `.swift-version` file is now deprecated.
```

Tested by running:

```
pod lib lint
```
material-foundation/material-testing-ios,material-foundation/material-testing-ios,material-foundation/material-testing-ios