Bump Jekyll to v2.5.0

Release post: http://jekyllrb.com/news/2014/11/05/jekylls-midlife-crisis-jekyll-turns-2-5-0/
GitHub Release: https://github.com/jekyll/jekyll/releases/tag/v2.5.0
Diff: https://github.com/jekyll/jekyll/compare/v2.4.0...v2.5.0

Security should be happy with this: all path sanitation is centralized now.
See https://github.com/jekyll/jekyll/pull/2882 for more info.

Also be sure to set `JEKYLL_NO_BUNDLER_REQUIRE=true` in your environment.

/cc @benbalter @mastahyeti @gjtorikian @gregose