Suggest "yarn build" rather than "yarn run build" (#1800)

* Fix for issue #1798: Suggested 'yarn build' versus 'yarn run build'

* remove 'run' from 'yarn test' command as well

* conditionally show 'run' if Yarn is not available

Author: geoffdavis92

packages/react-scripts/scripts/init.js

@@ -127,13 +127,13 @@ module.exports = function(appPath, appName, verbose, originalDirectory, template
   console.log(chalk.cyan('  ' + displayedCommand + ' start'));
   console.log('    Starts the development server.');
   console.log();
-  console.log(chalk.cyan('  ' + displayedCommand + ' run build'));
+  console.log(chalk.cyan(`  ${displayedCommand} ${useYarn ? '' : 'run '}build`));
   console.log('    Bundles the app into static files for production.');
   console.log();
   console.log(chalk.cyan('  ' + displayedCommand + ' test'));
   console.log('    Starts the test runner.');
   console.log();
-  console.log(chalk.cyan('  ' + displayedCommand + ' run eject'));
+  console.log(chalk.cyan(`  ${displayedCommand} ${useYarn ? '' : 'run '}eject`));
   console.log('    Removes this tool and copies build dependencies, configuration files');
   console.log('    and scripts into the app directory. If you do this, you can’t go back!');
   console.log();