Enable PHP 7.4 in CI (#102)

Author: vearutop

.travis.yml

@@ -1,6 +1,7 @@
 language: php
 php:
   - nightly
+  - 7.4
   - 7.3
   - 7.2
   - 7.1

CHANGELOG.md

@@ -4,6 +4,10 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [0.12.28] - 2020-01-27
+
+### Fixed
+- Trying to access array offset on value of type null in PHP 7.4, [#101](https://github.com/swaggest/php-json-schema/pull/101).
 
 ## [0.12.27] - 2020-01-26
 
@@ -45,6 +49,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Changed
 - Export `null` value instead of skipping it for properties having `null` type.
 
+[0.12.28]: https://github.com/swaggest/php-json-schema/compare/v0.12.27...v0.12.28
 [0.12.27]: https://github.com/swaggest/php-json-schema/compare/v0.12.26...v0.12.27
 [0.12.26]: https://github.com/swaggest/php-json-schema/compare/v0.12.25...v0.12.26
 [0.12.25]: https://github.com/swaggest/php-json-schema/compare/v0.12.24...v0.12.25