Update bin/spring with new Rails 6.1 syntax (#7288)

bin/spring

@@ -1,18 +1,16 @@
 #!/usr/bin/env ruby
 # frozen_string_literal: true
 
-# This file loads Spring without using Bundler, in order to be fast.
-# It gets overwritten when you run the `spring binstub` command.
-
-unless defined?(Spring)
-  require 'rubygems'
+if !defined?(Spring) && [nil, 'development', 'test'].include?(ENV['RAILS_ENV'])
+  gem 'bundler'
   require 'bundler'
 
-  lockfile = Bundler::LockfileParser.new(Bundler.default_lockfile.read)
-  spring = lockfile.specs.detect { |spec| spec.name == 'spring' }
-  if spring
+  # Load Spring without loading other gems in the Gemfile, for speed.
+  Bundler.locked_gems&.specs&.find { |spec| spec.name == 'spring' }&.tap do |spring|
     Gem.use_paths Gem.dir, Bundler.bundle_path.to_s, *Gem.path
     gem 'spring', spring.version
     require 'spring/binstub'
+  rescue Gem::LoadError
+    # Ignore when Spring is not installed.
   end
 end