Temp fix of forkme button width on rtfd.org

After the merge of #203, I noticed that the github ribbon on rtfd.org has a weird behavior when the width of the page is less than 768px. I guess it has to do sth with the media queries of the theme we are using, but I am not so familiar in order to fix it within the css. This PR is a temp fix, setting `max-width: 149px` inline the img source. 

## Before
![forkme_wrong](https://f.cloud.github.com/assets/340023/1612933/b175957c-55cd-11e3-9518-69da7db81cc1.png)

## After
![forkme_fix](https://f.cloud.github.com/assets/340023/1612932/b05f6afa-55cd-11e3-8cf3-b62d0626e8fe.png)