changes jekyll-octicons from optional to a dependency

[dmalan]

jekyll-octicons is included in PLUGIN_WHITELIST in https://github.com/github/pages-gem/pull/483/files (which is great!) but would just like to propose that it be made a dependency as well so that remote themes can utilize {% octicon %} without users having to know to add jekyll-octicons to their _config.yml as well. Else less-comfortable users who might be using GitHub's web UI to edit pages (and, in turn, to deploy to GitHub Pages) won't know why their use of the theme is failing, since they won't see Unknown tag 'octicon' included since they're not building locally.