Fixes [GH-654] on Windows 8x64 and Ruby 1.9.3p374

Replaces the command with absolute path version if it exists.

lib/vagrant/util/subprocess.rb

@@ -26,6 +26,11 @@ module Vagrant
       def initialize(*command)
         @options = command.last.is_a?(Hash) ? command.pop : {}
         @command = command
+        if Platform.windows?
+          locations = `where #{command[0]}`
+          new_command = "#{locations.split("\n")[0]}"
+          @command[0] = new_command if $?.success? and File.exists?(new_command)
+        end
         @logger  = Log4r::Logger.new("vagrant::util::subprocess")
       end