Force LESS regeneration when a variable change is detected

I have been experiencing an issue relating to the theme customiser. When making changes to colour etc the theme preview was not being updated.

It looks like the `$less->cachedCompile` method checks the file modification time and will only recompile if the files have actually changed. During theme customisation the files do not change but the variables passed into LESS do.

We can force a cachedCompile when a variable change is detect.

Let me know your thoughts ... less php should probably implement a fix for this longer term?