First, setup Bundler. Then, require it.

Per a conversation in
https://github.com/bundler/bundler/issues/3252#issuecomment-63208769

lib/jekyll/plugin_manager.rb

@@ -37,7 +37,8 @@ module Jekyll
         false
       else
         require "bundler"
-        required_gems = Bundler.require(:jekyll_plugins)
+        Bundler.setup # puts all groups on the load path
+        Bundler.require(:jekyll_plugins) # requires the gems in this group only
         Jekyll.logger.debug("PluginManager:", "Required #{required_gems.map(&:name).join(', ')}")
         ENV["JEKYLL_NO_BUNDLER_REQUIRE"] = "true"
         true