Add .nojekyll file to avoid Jekyll build errors

I noticed that some pull requests which were submitted to fix issues in http://whatisthor.com/ (the site generated from the `gh-pages` branch of [erikhuda/thor](https://github.com/erikhuda/thor)) weren't actually being published.

For example: https://github.com/erikhuda/thor/pull/356

![broken-link](https://cloud.githubusercontent.com/assets/65057/3320780/cad2c550-f72e-11e3-881c-d059b186920d.png)

That broken link still exists. :arrow_up:

This was being caused by Jekyll build failures, when attempting to build the contents of this repository. This repository uses `nanoc` to generate static content which is pushed, rather than relying on the Jekyll build on the GitHub Pages side, so this pull request [switches off Jekyll processing](https://help.github.com/articles/using-jekyll-with-pages#turning-jekyll-off) for pushes to `gh-pages` by adding a `.nojekyll` file.

If merged, this will allow the existing `rake deploy` task to be used to publish updates. :v: 

/cc @sferik @wycats