Check that Bundler is really set up before loading plugins with it

Requiring 'bundler/setup' won't do much if the Gemfile is not found,
leaving `Bundler.require` undefined.

bin/tf

@@ -1,6 +1,6 @@
 #!/usr/bin/env ruby
 
-require 'bundler/setup' if File.exist? File.expand_path('../../Gemfile', __FILE__)
+require 'bundler/setup' if File.exist?(File.expand_path('../../Gemfile', __FILE__))
 require 'yle_tf/cli'
 
 cli = YleTf::CLI.new(ARGV)

lib/yle_tf/plugin/loader.rb

@@ -19,10 +19,10 @@ def self.load_core_plugins
       end
 
       def self.load_bundler_plugins
-        if defined?(Bundler)
-          print_bundler_plugin_list if Logger.debug?
-          Bundler.require(BUNDLER_PLUGIN_GROUP)
-        end
+        return if !bundler_set_up?
+
+        Logger.debug { print_bundler_plugin_list }
+        Bundler.require(BUNDLER_PLUGIN_GROUP)
       end
 
       def self.load_user_plugins
@@ -47,6 +47,10 @@ def self.user_plugins
         ENV.fetch('TF_PLUGINS', '').split(/[ ,]+/)
       end
 
+      def self.bundler_set_up?
+        defined?(Bundler) && Bundler.respond_to?(:require)
+      end
+
       def self.print_bundler_plugin_list
         plugins = bundler_plugins
         if !plugins.empty?