Travis CI tests broken due to change in MacOSX image

Description

Travis CI tests broken due to change in MacOSX image. The error message is as follows:

❌ “Use Legacy Swift Language Version” (SWIFT_VERSION) is required to be configured correctly for targets which use Swift. Use the [Edit > Convert > To Current Swift Syntax…] menu to choose a Swift version or use the Build Settings editor to configure the build setting directly.

▸ Building Pods/RxSwift [Debug]
▸ Check Dependencies

❌ “Use Legacy Swift Language Version” (SWIFT_VERSION) is required to be configured correctly for targets which use Swift. Use the [Edit > Convert > To Current Swift Syntax…] menu to choose a Swift version or use the Build Settings editor to configure the build setting directly.

Testing failed:
“Use Legacy Swift Language Version” (SWIFT_VERSION) is required to be configured correctly for targets which use Swift. Use the [Edit > Convert > To Current Swift Syntax…] menu to choose a Swift version or use the Build Settings editor to configure the build setting directly.
** TEST FAILED **

The following build commands failed:
Check dependencies
Check dependencies
(2 failures)

For full log, please visit https://travis-ci.org/swagger-api/swagger-codegen/builds/286084607

Swagger-codegen version

Latest master

Suggest a fix/enhancement

Nothing changed in our end. The error is due to changes in the build environment.

We may want to try using another build image xcode9.1 to test instead.

Ref: https://docs.travis-ci.com/user/languages/objective-c/

Start with the Travis CI configuration and the linked build log, then check which macOS/Xcode image is used for the Swift targets. Test the proposed xcode9.1 image, or the project's supported alternative, and confirm that the Travis job completes without the SWIFT_VERSION dependency errors.