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

I tried to use the [vagrant-digitalocean](https://github.com/smdahlen/vagrant-digitalocean) plugin and I experienced the same problem what is described in this issue.

After a few hours of investigation I found that vagrant can't locate the rsync command because the extension is not given. When I modified the command name to the extension postfixed version (e.g. rsync.bat or rsync.exe), then Vagrant could run the command.

Windows provides a command which is similar to the 'which' command under Linux. It is called "where". I added a few lines of code that tempt to locate the absolute path of the command and if this operation is successful then it replaces the command.