Add rouge as an alternative to pygments (Jekyll 2.x)

Jekyll 2 supports https://github.com/jneen/rouge hightlighter as an alternative to pygments.
This change adds current version of rouge (1.5.1) to github-pages.

**Backlog and known issues:**
- [ ] #75 Upgrade to Jekyll 2.1
- [ ] jekyll/jekyll#2607 Invalid highlight html with rouge and linenos