use compatible versions of gems

- json: "~> 2.0"
    # https://github.com/flori/json/issues/303#issuecomment-259426660

  - pygments.rb: "~> 1.1"

Gemfile

@@ -3,6 +3,13 @@ gemspec :name => "jekyll"
 
 gem "rake", "~> 12.0"
 
+if RUBY_VERSION >= '2.4'
+  gem "json", "~> 2.0"
+  gem "pygments.rb", "~> 1.1"
+else
+  gem "pygments.rb", "~> 0.6.0" unless RUBY_ENGINE == "jruby"
+end
+
 # Dependency of jekyll-mentions. RubyGems in Ruby 2.1 doesn't shield us from this.
 gem "activesupport", "~> 4.2", :groups => [:test_legacy, :site] if RUBY_VERSION < "2.2.2"
 
@@ -75,7 +82,6 @@ group :jekyll_optional_dependencies do
   platform :ruby, :mswin, :mingw, :x64_mingw do
     gem "classifier-reborn", "~> 2.0"
     gem "liquid-c", "~> 3.0"
-    gem "pygments.rb", "~> 0.6.0"
     gem "rdiscount", "~> 2.0"
     gem "redcarpet", "~> 3.2", ">= 3.2.3"
   end