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

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

Precursor to #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
```

Author: featherless

Diff for: MDFTesting.podspec

@@ -10,6 +10,7 @@ Pod::Spec.new do |s|
   s.requires_arc = true
   s.ios.deployment_target = '9.0'
   s.default_subspec = "MDFTesting"
+  s.swift_version = '5.0'
 
   s.subspec "MDFTesting" do |ss|
     ss.public_header_files = "src/*.h"