Mark certain gems as eventually-non-core.

jekyll.gemspec

@@ -28,13 +28,16 @@ Gem::Specification.new do |s|
   s.rdoc_options = ["--charset=UTF-8"]
   s.extra_rdoc_files = %w[README.markdown LICENSE]
 
-  s.add_runtime_dependency('liquid', "~> 2.6.1")
-  s.add_runtime_dependency('classifier', "~> 1.3")
-  s.add_runtime_dependency('kramdown', "~> 1.3")
-  s.add_runtime_dependency('pygments.rb', "~> 0.6.0")
+  s.add_runtime_dependency('liquid',    "~> 2.6.1")
+  s.add_runtime_dependency('kramdown',  "~> 1.3")
   s.add_runtime_dependency('mercenary', "~> 0.3.3")
   s.add_runtime_dependency('safe_yaml', "~> 1.0")
   s.add_runtime_dependency('colorator', "~> 0.1")
+
+  # Before 3.0 drops, phase the following gems out as dev dependencies
+  # and gracefully handle their absence.
+  s.add_runtime_dependency('classifier', "~> 1.3")
+  s.add_runtime_dependency('pygments.rb', "~> 0.6.0")
   s.add_runtime_dependency('redcarpet', "~> 3.1")
   s.add_runtime_dependency('toml', '~> 0.1.0')
   s.add_runtime_dependency('jekyll-paginate', '~> 1.0')

lib/jekyll.rb

@@ -27,7 +27,6 @@ require 'logger'
 require 'liquid'
 require 'kramdown'
 require 'colorator'
-require 'toml'
 
 # internal requires
 require 'jekyll/version'
@@ -74,8 +73,15 @@ require_all 'jekyll/tags'
 # plugins
 require 'jekyll-coffeescript'
 require 'jekyll-sass-converter'
-require 'jekyll-paginate'
-require 'jekyll-gist'
+
+# Eventually remove these for 3.0 as non-core
+require "classifier"
+require "pygments.rb"
+require "toml"
+require "jekyll-paginate"
+require "jekyll-gist"
+require "jekyll-coffeescript"
+require "jekyll-sass-converter"
 
 SafeYAML::OPTIONS[:suppress_warnings] = true