Fix "cd: command not found" error when executing tasks with sudo

When **use_sudo** is set to **true** in Capistrano config, executing any Grunt task produces the following error:
```
 * <[33mexecuting "sudo -p 'sudo password: ' cd /vagrant/releases/20130831114831 && grunt  default"<[0m
   servers: ["127.0.0.1"]
   [127.0.0.1:2222] executing command
** [out :: 127.0.0.1:2222] sudo: cd: command not found
```
Pull request fixes that.