Update last-modified for AssetCache asset

This feature will allow to fix LESS and SASS filters caching problems when using @import (see #127 and #169)

And after changes in AsseticController similar with https://github.com/covex-nn/AsseticBundle/commit/8cd3382c2356718d3febdb214be045c0510f041d there won't be any problems with caching @imports