Bump Jekyll to make localhost site look as city.opendata.by

[abitrolly]

Web site sources specify Jekyll 3.6.3

https://github.com/opendataby/city-dashboard/blob/88e314022a1c57e437a9c6e0f6f9dcb8a6ec69ed/Gemfile#L12

But GitHub uses Jekyll 3.8.5 to render it! See https://city.opendata.by/about/ (version info added in ee0008d7ee8964d100a0e54adcdbfd5f23b30b72).

[abitrolly]

The actual problem was not the outdated Jekyll version, but the fact that contents on https://city.opendata.by is different from local version. This is how it looks at http://localhost:4000/

And this is what https://city.opendata.by/ shows.

[abitrolly]

The problem is most likely in https://github.com/benbalter/jekyll-optional-front-matter plugin, which makes any markdown file a website page. Without this plugin Jekyll only processes files with front-matter.