Updating image to align with marketing

updating the buildpack image to align better with marketing (and fix the code block in the background)

README.md

 # Heroku Buildpack for Node.js
 
-![nodesjs](https://cloud.githubusercontent.com/assets/51578/8882955/3f0c3980-3219-11e5-8666-bc9c926a7356.jpg)
+![nodejs](https://cloud.githubusercontent.com/assets/51578/13712672/efdf2a40-e792-11e5-82ef-492478cbc0dc.png)
 
 This is the official [Heroku buildpack](http://devcenter.heroku.com/articles/buildpacks) for Node.js apps.