Fixing Warning: exec() [<a href='function.exec'>function.exec</a>]: Unable to fork [df -T -t nfs reported in http://forum.piwik.org/read.php?2,99550

git-svn-id: http://dev.piwik.org/svn/trunk@7768 59fd770c-687e-43c8-a1e3-f5a4ff64c105