Vapor build failure

I am using Mac with Sierra OS (10.12.5). I Installed Vapor using home-brew. If I tried to run command 'vapor build' in terminal it shows following error.

No .build folder, fetch may omegle.2yu.co take a while... Fetching Dependencies [ • ] Error: backgroundExecute(code: 1, error: "swift-package: error: unknown command: --enable-prefetching\n", output: "") Please help https://omglz.com me.