refs #4373 I had to make this change to force reloading the css in case the compiled css changes without a version change

Following scenario:
There is a plugin that generates css/less files dynamically from time to time but the Plugin version does not change as there is no plugin update. Only the less file changes. When the less file changes, the plugin deletes the merged asset file as well.

The current cache buster is based on Piwik version + Plugin versions. That means, when a less file changes but no version the browser will still use the "cached CSS" as the cache buster does not change. 

Therefore I have to build a cacheBuster value based on the content of the compiled CSS file to invalidate the cached CSS file in the browser and to force the reloading. As far as I can see this change does not have any huge performance implications as it builds a hash based on the already compiled CSS file and only tries to regenerate the compiled CSS file if it does not exist.