docs: update headless chrome/puppeteer example

Puppeteer v0.11 has released a new method to obtain the locally
installed chromium path. This commit cleans up the code example to use
that new method.

README.md

@@ -66,12 +66,7 @@ $ npm install puppeteer --save-dev
 
 ```js
 // karma.conf.js
-
-const ChromiumRevision = require('puppeteer/package.json').puppeteer.chromium_revision
-const Downloader = require('puppeteer/utils/ChromiumDownloader')
-const revisionInfo = Downloader.revisionInfo(Downloader.currentPlatform(), ChromiumRevision)
-
-process.env.CHROME_BIN = revisionInfo.executablePath
+process.env.CHROME_BIN = require('puppeteer').executablePath()
 
 module.exports = function(config) {
   config.set({