Bump yiisoft/yii2 from 2.0.49.1 to 2.0.50 in /core

Bumps [yiisoft/yii2](https://github.com/yiisoft/yii2-framework) from 2.0.49.1 to 2.0.50.
- [Changelog](https://github.com/yiisoft/yii2-framework/blob/master/CHANGELOG.md)
- [Commits](yiisoft/yii2-framework@2.0.49.1...2.0.50)

---
updated-dependencies:
- dependency-name: yiisoft/yii2
  dependency-type: direct:production
...

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