Update from Ruby 2.0->2.5

- Some dependency updates required

Gemfile

@@ -5,7 +5,7 @@ source 'https://rubygems.org'
 ## version. Then remind one of the site maintainers that they need to
 ## run `rvm install <VERSION>` on the build server(s) before they commit
 ## to master
-ruby '2.0.0'
+ruby '2.5.0'
 
 ## Used on the build server. If you add a package here (like nokogiri)
 ## that has non-Gem dependencies (like zlib), please remind the site
@@ -17,7 +17,7 @@ group :development do
   ## When we upgrade to Jekyll 3.0.0 or higher, remove
   ## _plugin/remove-html-extension.rb
   gem 'jekyll', '~>3.0'
-  gem 'json'
+  gem 'json', '>=1.9'
   gem 'less', '2.4.0'
   gem 'kramdown'
   gem 'RedCloth'