lessphp: Update to upstream 6e8e724fc7

Summary of changes:

- lessphp throws errors when referencing undeclared variables and mixins.
- lessc::addParsedFile is now public.

See <https://github.com/leafo/lessphp/compare/785ad53840...6e8e724fc7>.

Change-Id: Idffe6946f4029659e677f9dfb62566096cc8d5b5