* 2.3:
Fixed compatibility with PHP7 and up by introducing new constraints (IsNull, IsTrue, IsFalse) and related validators (IsNullValidator, IsTrueValidator, IsFalseValidator)
Conflicts:
src/Symfony/Component/Form/composer.json
src/Symfony/Component/Validator/CHANGELOG.md
src/Symfony/Component/Validator/Constraints/FalseValidator.php
src/Symfony/Component/Validator/Constraints/NullValidator.php
src/Symfony/Component/Validator/Constraints/TrueValidator.php
src/Symfony/Component/Validator/Tests/Constraints/IsFalseValidatorTest.php
src/Symfony/Component/Validator/Tests/Constraints/IsNullValidatorTest.php
src/Symfony/Component/Validator/Tests/Constraints/IsTrueValidatorTest.php