laminas-captcha 2.8.0

Based on https://github.com/zendframework/zend-captcha/releases/tag/release-2.8.0 (commit 521069d398e1ad28e30d15b6b5e1638a1b90a467 in this repo)

Added
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- [zendframework/zend-captcha#39](https://github.com/zendframework/zend-captcha/pull/39) adds support for PHP 7.1 and 7.2.

Changed
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- Nothing.

Deprecated
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- Nothing.

Removed
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- [zendframework/zend-captcha#39](https://github.com/zendframework/zend-captcha/pull/39) removes support for HHVM.

Fixed
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- [zendframework/zend-captcha#23](https://github.com/zendframework/zend-captcha/pull/23) fixes an issue with garbage collection of expired CAPTCHA images
  when concurrent requests trigger collection.

- [zendframework/zend-captcha#31](https://github.com/zendframework/zend-captcha/pull/31) fixes using the
  ReCaptcha response as the value parameter to isValid().