Issue #2172235 by joelpittet: Upgrade Twig to 1.15.* from 1.12.*.

composer.json

composer.lock

core/vendor/twig/twig/.travis.yml

core/vendor/twig/twig/AUTHORS

core/vendor/twig/twig/CHANGELOG

core/vendor/twig/twig/README.rst

core/vendor/twig/twig/composer.json

core/vendor/twig/twig/doc/advanced.rst

core/vendor/twig/twig/doc/advanced_legacy.rst

core/vendor/twig/twig/doc/api.rst

core/vendor/twig/twig/doc/coding_standards.rst

core/vendor/twig/twig/doc/deprecated.rst

core/vendor/twig/twig/doc/filters/abs.rst

core/vendor/twig/twig/doc/filters/batch.rst

core/vendor/twig/twig/doc/filters/convert_encoding.rst

core/vendor/twig/twig/doc/filters/date.rst

core/vendor/twig/twig/doc/filters/date_modify.rst

core/vendor/twig/twig/doc/filters/default.rst

core/vendor/twig/twig/doc/filters/escape.rst

core/vendor/twig/twig/doc/filters/format.rst