don't specify pre/tt/code font-size for gh-pages

Makes [the introduction page](http://pivotal.github.io/jasmine/) a little more readable.

before:

![gh-pages-before](https://f.cloud.github.com/assets/174307/857700/6814755a-f557-11e2-802f-eba0ee6aba57.png)

after:

![gh-pages-after](https://f.cloud.github.com/assets/174307/857702/6d136a8e-f557-11e2-8ce3-e995a40f431a.png)

(Chrome on OS X)