Foodcritic Update, FC001

Updated Foodcritic to 4.0.0, since it was supposed to run faster. While reading the Changelog, I stumbled upon the remarks that usage of symbols can under some circumstances lead to memory leaks in chef, so I fixed FC001 and moved everything to strings. 

Then, I realized that this warning only applies to ruby enterprise edition. I also could not determine a significant speed up with Foodcritic 4. So this PR is kind of stupid from a technical point of view, it just fixes a small typo in percona-xtrabackup while I was checking the diff.

But at least we are now a bit more standardized.

![](http://s3.amazonaws.com/design_images/images/2930/it_ssomething.png)