DP: gem update

[inertia186]

We're currently on jekyll-3.3.1. Recommend upgrade to jekyll-3.8.4.

https://jekyllrb.com/docs/history/

We should modify Gemfile and remove the preferred versions from all jekyll gems, so we end up with:

source 'https://rubygems.org'

gem 'jekyll'
gem 'neatjson'
gem 'rake'
gem 'git'
gem 'steem-ruby'

group :jekyll_plugins do
  gem 'jekyll-seo-tag'
  gem 'jekyll-sitemap'
end

Then, we can execute:

$ bundle update

Which locks all of the related gems to the latest versions:

Installing jekyll-watch 2.0.0 (was 1.5.1)
Installing liquid 4.0.1 (was 3.0.6)
Installing rouge 3.3.0 (was 1.11.1)
Installing jekyll 3.8.4 (was 3.3.1)
Installing jekyll-seo-tag 2.5.0 (was 2.1.0)
Installing jekyll-sitemap 1.2.0 (was 0.12.0)

AC

• [x] devportal upgraded to jekyll-3.8.4
• [x] check entire site problems after upgrade