[SECURITY] Update phpunit/phpunit from 5.5.7 to 5.7.27

[violinist-bot]

If you have a high test coverage index, and your tests for this pull request are passing, it should be both safe and recommended to merge this update.

Updated packages

Some times an update also needs new or updated dependencies to be installed. Even if this branch is for updating one dependency, it might contain other installs or updates. All of the updates in this branch can be found here:

• phpdocumentor/reflection-docblock: 3.3.2 (updated from 3.2.2)
• phpdocumentor/type-resolver: 0.4.0 (updated from 0.3.0)
• phpspec/prophecy: v1.10.3 (updated from v1.7.2)
• phpunit/php-file-iterator: 1.4.5 (updated from 1.4.2)
• phpunit/php-token-stream: 1.4.12 (updated from 1.4.11)
• phpunit/phpunit: 5.7.27 (updated from 5.5.7)
• sebastian/code-unit-reverse-lookup: 1.0.2 (updated from 1.0.1)
• sebastian/exporter: 2.0.0 (updated from 1.2.2)
• sebastian/object-enumerator: 2.0.1 (updated from 1.0.0)
• sebastian/recursion-context: 2.0.0 (updated from 1.0.5)
• symfony/polyfill-ctype: v1.19.0 (new package, previously not installed)
• symfony/yaml: v3.3.18 (updated from v3.3.10)
• webmozart/assert: 1.9.1 (updated from 1.2.0)

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