text: Title: Link to "Edit in Github" still broken on project homepage
  Body: This is a continuation of issue #146. The link seems to still be broken.
  
  # Steps to Repro:
  
  - Go to the project homepage http://drivendata.github.io/cookiecutter-data-science/
  - In the top right there is a button "Edit on Github" that links to this page: https://github.com/drivendata/cookiecutter-data-science/edit/master/docs/index.md
  - Click on that link
  
  # What I got
  
  The link sends me to a 404 "not found" error page on github.
  
  # What I wanted
  
  What I expected was it would send me to some page on GitHub.
  
  # Possible fix
  
  I imagine maybe the docs is built from the gh-pages branch and not the master branch - if that's the case we would need to edit [this line spefically](https://github.com/drivendata/cookiecutter-data-science/blob/9e01bf8d09c6dd65f435acc50444971b771ebfe4/index.html#L74) on the gh-pages branch.
  