Update lessphp to upstream head (785ad53840)

This patch updates lessc.inc.php to 785ad53840:
<https://github.com/leafo/lessphp/commit/785ad53840b1777ef2120a10dcb2a5d5e6699f36>.

Differences from our version:

- Updated year in copyright notice
- Added support for 'precision' parameter to round().

Change-Id: If22afea96741c6d8fedefc05c49b24ae6e7809b0