this drops support for swift <4.1 and with this support for xcode <=9.2.
this was the last setup that is officially working on macOS 10.12.
our old legacy build macOS 10.12 + xcode 9.2 is replaced by macOS 10.13
+ xcode 9.4.1 with swift 4.1. the macOS 10.13 + xcode 10.1 VM is
replaced by the latest macOS 10.14 + xcode 11.3.1 VM. this is the oldest
version officially supported by Apple.
this is in preparations for the following commit.