Only use shell_exec if it's available

We should only be using `shell_exec` if it's available:

````
[21-Jan-2014 00:04:13 America/Chicago] PHP Warning: shell_exec() has been disabled for security reasons in /home/username/public_html/wp-content/plugins/backupwordpress/classes/class-requirements.php on line 254
````

We can use [`HM_Backup:: is_shell_exec_available()`](https://github.com/humanmade/hm-backup/blob/master/hm-backup.php#L175)