Fixed performance regression in "watch" and dev-server

Hi Webpackers!

This release fixes a performance regression when using watch or dev-server. In both cases, every "re-build" would take as long as the original build. See #905 for the fix.

yarn upgrade "@symfony/webpack-encore@^1.0.3"

Changes: v1.0.2..v1.0.3

Documentation: http://symfony.com/doc/current/frontend.html

Happy packing!