mkdir race condition

I have added @ to mkdir on line 41 to avoid race condition when showing many image with picnic for the first time. Without this, I'm getting internal server error with error:

'ErrorException' with message 'mkdir(): File exists' in /var/www/app.dev/vendor/mjolnic/picnic/src/Mjolnic/Picnic/Server.php:41
Merge pull request #1 from dragosmocrii/patch-1

mkdir race condition