Dependency on XCode

After first run on clean macOS High Sierra installation scripts says that 'Command Line Tools' must be installed first and exits.

May be we can add installation of 'command line tools' as very first step.

Also, when script installs first of gems with extentions (and run clang compiler) script fails because user must accept XCode license before can use compiler. Because of this user must re-run script after some manual actions with XCode.