Replace all instances of 'pace-done' within className

Sometimes pace suddenly runs twice and so adds `pace-done` to the `<body>` twice. This PR is to prevent such things to happen.

I couldn't find any issue that would force this behavior. I have no custom options but I load pace with $script.js and am tracking events triggered by AngularJS.

BTW is there a reason for replacing the class with an empty string and appending another string afterwards instead of replacing the class with the new class?