Issue #2712637 by klausi, jibran, dawehner, simensen, JeroenT: Update stack/builder for Symfony 3 compatibility

Issue #2638856 by bojanz, webflo, SiliconMind: Unable to install modules with core dependencies via composer

Issue #2678574 by dawehner: Symfony 2.8 requirements change: required PHP "iconv" extension is nowhere documented

Issue #2550519 by klausi, nullkernel, cweagans, alexpott, pwolanin, neclimdul: Crypt::randomBytes() doesn't actually return cryptographically secure random bytes

Issue #2600176 by alexpott, dawehner, tim.plunkett: Step 1.875: Include the PhantomJS Mink driver for real

Issue #2380389 by webflo, Mile23, davidwbarratt, timmillwood, hussainweb, tstoeckler, bojanz, yched, MKorostoff, alexpott, webchick: Use a single vendor directory in the root

Issue #2571817 by Cottser, hussainweb: Remove race condition handling in TwigEnvironment::loadTemplate(), it's fixed upstream now

Issue #2568171 by Cottser, dawehner, hussainweb, Wim Leers, cilefen, znerol: Upgrade to Twig 1.22 and implement our own cache class

Issue #2568085 by plach, alexpott, dawehner, Berdir: Lock Twig on ~1.21.2 until we are able to upgrade to 1.22.0

Issue #2444615 by borisson_, neclimdul, jibran, dawehner, jhedstrom, Fabianx: Move testing packages to require-dev in composer.json

Issue #2546618 followup by plach: Document that we don't depend on the Symfony Translation component.

Issue #2546618 by plach, dawehner, chx, bojanz: Replace the Symfony Translation component with a custom interface since we don't use it