Several reports of issues with 3.0.2

````
Warning: exec() has been disabled for security reasons in /data/web/virtuals/51126/virtual/www/domains/trickoskockou.cz/wp-content/plugins/backupwordpress/functions/interface.php on line 106

Warning: exec() has been disabled for security reasons in /data/web/virtuals/51126/virtual/www/domains/trickoskockou.cz/wp-content/plugins/backupwordpress/functions/interface.php on line 107
````
https://wordpress.org/support/topic/update-to-version-302-problem?replies=12#post-6290183

My guess is that this code was never executed in previous versions, at least 2.6.2. `hmbkp_path()` always returns a valid directory.

https://github.com/humanmade/backupwordpress/blob/2.6.2/functions/interface.php#L58