Update Environment#dotfile_path to take advantage of new Pathname object

lib/vagrant/environment.rb

@@ -55,7 +55,7 @@ module Vagrant
     # The path to the `dotfile`, which contains the persisted UUID of
     # the VM if it exists.
     def dotfile_path
-      root_path ? File.join(root_path, config.vagrant.dotfile_name) : nil
+      root_path.join(config.vagrant.dotfile_name) rescue nil
     end
 
     # The path to the home directory, which is usually in `~/.vagrant/~

test/vagrant/environment_test.rb

@@ -54,7 +54,7 @@ class EnvironmentTest < Test::Unit::TestCase
 
     context "dotfile path" do
       should "build up the dotfile out of the root path and the dotfile name" do
-        assert_equal File.join(@env.root_path, @env.config.vagrant.dotfile_name), @env.dotfile_path
+        assert_equal @env.root_path.join(@env.config.vagrant.dotfile_name), @env.dotfile_path
       end
     end