Update index.js

When check() fails a error occurs. It seems to be fixed with this small change. Unit tests would be good ...

ReferenceError: defaults is not defined
    at /Users/heideggermartin/IdeaProjects/mapslice/node_modules/yargs/index.js:339:46
    at Array.forEach (native)
    at Object.Argv.self.help (/Users/heideggermartin/IdeaProjects/mapslice/node_modules/yargs/index.js:302:14)
    at Object.Argv.self.showHelp (/Users/heideggermartin/IdeaProjects/mapslice/node_modules/yargs/index.js:241:17)
    at fail (/Users/heideggermartin/IdeaProjects/mapslice/node_modules/yargs/index.js:168:18)
    at /Users/heideggermartin/IdeaProjects/mapslice/node_modules/yargs/index.js:420:17
    at Array.forEach (native)
    at parseArgs (/Users/heideggermartin/IdeaProjects/mapslice/node_modules/yargs/index.js:410:16)
    at Object.Argv.Object.defineProperty.get [as argv] (/Users/heideggermartin/IdeaProjects/mapslice/node_modules/yargs/index.js:368:36)
    at Object.<anonymous> (/Users/heideggermartin/IdeaProjects/mapslice/bin/mapslice:24:46)