Use File.exist? instead of File.exists? (#334)

In newer versions of Ruby using File.exists? will warn about deprecation. The
File.exist? method has been around forever so this will not introduce any
backwards compatibility problems.

example/rails-32/config/boot.rb

@@ -3,4 +3,4 @@
 # Set up gems listed in the Gemfile.
 ENV['BUNDLE_GEMFILE'] ||= File.expand_path('../../Gemfile', __FILE__)
 
-require 'bundler/setup' if File.exists?(ENV['BUNDLE_GEMFILE'])
+require 'bundler/setup' if File.exist?(ENV['BUNDLE_GEMFILE'])