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

packages/react-scripts/scripts/init.js

@@ -149,13 +149,17 @@ module.exports = function(
   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'
   );