This caused a conflict in the language objectification patch. Postponing.

This reverts commit 9e017ada.

Issue #2151829 by dawehner, chx: Doctrine annotation parsing takes an unacceptable amount of time/memory on install.

Issue #1839468 by ParisLiakos, damiankloip, dawehner, jibran: Replace aggregator rss parsing with Zend Feed.

Issue #1894002 by scor, msonnabaum, effulgentsia: Update vendor libraries and pin them to specific versions in composer.json.

Issue #1849564 by attiks, Rob Loach, fago: Add the symfony validator component to core despite Symfony potentially releasing BC-breaking updates after 2.3.

Caused a conflict with a higher-priority patch; temporary rollback for now.

This reverts commit ba6d3fb7.

Issue #1834594 by effulgentsia, Rob Loach: Update dependencies (Symfony and Twig) follow up fixes for Composer.

Issue #1424924 by Rob Loach: Added Use Composer for updating Symfony components (without removing Symfony code from repo).