Ensure bundler is required when used

Spring.gemfile is using bundler, the require is needed in certain
cases in which Bundler is not already loaded.

Specifically when spring is loaded via Spring::ApplicationManager
the #start_child method wraps the call inside Bundler.with_original_env
restoring a RUBYOPT env variable without the bundler require. That in
turn causes errors like:

/Users/elia/.asdf/installs/ruby/2.7.1/lib/ruby/gems/2.7.0/gems/spring-2.1.1/lib/spring/configuration.rb:10:in `gemfile': uninitialized constant #<Class:Spring>::Bundler (NameError)

Author: elia

lib/spring/configuration.rb

@@ -1,4 +1,5 @@
 require "spring/errors"
+require "bundler"
 
 module Spring
   class << self