Log Message:
Update ruby-jekyll to 3.5.2.

3.5.2 (2017/8/18)

* Backport #6281 for v3.5.x: Fix Drop#key? so it can handle a nil argument (#6288)
* Backport #6280 for v3.5.x: Guard against type error in absolute_url (#6287)
* Backport #6266 for v3.5.x: Memoize the return value of Document#url (#6301)
* Backport #6273 for v3.5.x: delegate StaticFile#to_json to StaticFile#to_liquid \ 
(#6302)
* Backport #6226 for v3.5.x: Reader#read_directories: guard against an entry not \ 
being a directory (#6304)
* Backport #6247 for v3.5.x: kramdown: symbolize keys in-place (#6303)

3.5.1 (2017/7/18)

Minor Enhancements

* Use Warn for deprecation messages (#6192)
* site template: Use plugins key instead of gems (#6045)

Bug Fixes

* Backward compatiblize URLFilters module (#6163)
* Static files contain front matter default keys when to_liquid'd (#6162)
* Always normalize the result of the relative_url filter (#6185)

Documentation

* Update reference to trouble with OS X/macOS (#6139)
* added BibSonomy plugin (#6143)
* add plugins for multiple page pagination (#6055)
* Update minimum Ruby version in installation.md (#6164)
* [docs] Add information about finding a collection in site.collections (#6165)
* Add {%raw%} to Liquid example on site (#6179)
* Added improved Pug plugin - removed 404 Jade plugin (#6174)
* Linking the link (#6210)
* Small correction in documentation for includes (#6193)
* Fix docs site page margin (#6214)

Development Fixes

* Add jekyll doctor to GitHub Issue Template (#6169)
* Test with Ruby 2.4.1-1 on AppVeyor (#6176)
* set minimum requirement for jekyll-feed (#6184)