join the command into a string before executing

In a nutshell, `exec` blows up when you pass an array containing elements with spaces in them. This can come up when giving the `extra` arg:

```puppet
repository { "/tmp/repo":
  source  => "whatever/stuff",
  extra   => "-b my_cool_branch"
}
```

See boxen/puppet-ruby#19 for wayyyyy more detail.