Bump Jekyll to v2.5.2

v2.5.1 was a path sanitation bug fix for Windows users.
v2.5.2 was a fix for that fix. Fun, right?

v2.5.1 diff: https://github.com/jekyll/jekyll/compare/v2.5.0...v2.5.1
v2.5.1 release: https://github.com/jekyll/jekyll/releases/tag/v2.5.1

v2.5.2 diff: https://github.com/jekyll/jekyll/compare/v2.5.1...v2.5.2
v2.5.2 release: https://github.com/jekyll/jekyll/releases/tag/v2.5.2