CLI should not allow execution of iOS commands with Xcode8

[Natalia-Hristova]

Please, provide the details below:

Did you verify this is a real problem by searching the NativeScript Forum and the other open issues in this repo?

Tell us about the problem

CLI have to prevent users to execute ios commands when Xcode version is 8. tns-ios from version 4.2.0 is built with Xcode9 and this makes build/run ios etc. not possible with Xcode8. Also documentation says:
https://docs.nativescript.org/start/ns-setup-os-x

For iOS development:
Latest Xcode
...

Which platform(s) does your issue occur on?

iOS

Please provide the following version numbers that your issue occurs with:

• CLI: 4.2.3
• Runtime(s): "tns-ios" 4.2.0

Please tell us how to recreate the issue in as much detail as possible.

Execute tns build ios on mac with Xcode8.

Is there code involved? If so, please share the minimal amount of code needed to recreate the problem.

Consider using the playground to provide the code you have issues with.