Change contributions URLS to better ones

Use the rawgit.com instead of htmlpreview which works more reliable
(referenced in underscore doc links)

CONTRIBUTING.md

@@ -33,9 +33,9 @@ The frontend is really standalone code, but is integrated with/served by the Rai
 CartoDB is built on top of [CartoDB.js](https://github.com/CartoDB/cartodb.js),
 which in turns depends on some common libraries, in particular worth mentioning:
 
- - [BackboneJS 0.9.2](http://htmlpreview.github.io/?https://raw.github.com/jashkenas/backbone/0.9.2/index.html).
+ - [BackboneJS 0.9.2](https://cdn.rawgit.com/jashkenas/backbone/0.9.2/index.html).
  - [jQuery 1.7.2](http://api.jquery.com/category/version/1.7/)
- - [underscore.js 1.4.4](http://htmlpreview.github.io/?https://raw.github.com/jashkenas/underscore/1.4.4/index.html)
+ - [underscore.js 1.4.4](https://cdn.rawgit.com/jashkenas/underscore/1.4.4/index.html)
 
 Source code is located at `lib/assets/javascripts`, dependencies at `vendor/assets/javascripts`.