Make it work with current gh-pages/jekyll

The current gh-pages branch is broken on [GitHub Pages](http://redwallhp.github.io/solar-theme-jekyll/). This patchset makes it render again with current version of Jekyll and GitHub Pages settings. Also allows testing the site locally with `jekyll serve --baseurl ''` before pushing to GitHub. Tested on GitHub Pages [here](http://gizero.github.io/solar-theme-jekyll/).