NativeScript setup script fails on Mac

[tzraikov]

Setup script we promote on docs.nativescript.org does not work on macOS.

brew install Android Tools 25, while we create emulators with old command:
https://github.com/NativeScript/nativescript-cli/blob/master/setup/native-script.rb#L151

[dtopuzov]

Brew formula is also changed.
brew install android-sdk should be replaced with brew cask install android-sdk.