scripts/test: Use maxWorkers=4 when running on CI

It seems that Jest runs much faster in Travis when it's ran with this
flag.  E.g. for one particular project, this made the frontend testing
time drop from 20 minutes to 2 minutes.

packages/react-scripts/scripts/test.js

@@ -32,6 +32,9 @@ const argv = process.argv.slice(2);
 if (!process.env.CI && argv.indexOf('--coverage') < 0) {
   argv.push('--watch');
 }
+else {
+  argv.push('--maxWorkers=4');
+}
 
 // @remove-on-eject-begin
 // This is not necessary after eject because we embed config into package.json.