Consider swapping placehold.it for static images

Created by: macik

While reading «Documents» pages for bootstrap found broken links for some images. After investigation I discovered that «http://placehold.it/» block access for some countries (in my case is Russia) by IP's.
Message from site: «Your IP address based on the country, region or network has been flagged by the website owner.»

So many twitter examples are breaked.

As a suggestion - change it to other dump image service, like: http://dummyimages.com