Update to drupal 8.0.0-rc1. For more information, see https://www.drupal.org/node/2582663

vendor/phpspec/prophecy/CONTRIBUTING.md

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+Contributing
+------------
+
+Prophecy is an open source, community-driven project. If you'd like to contribute,
+feel free to do this, but remember to follow this few simple rules:
+
+- Make your feature addition or bug fix,
+- Add either specs or examples for any changes you're making (bugfixes or additions)
+  (please look into `spec/` folder for some examples). This is important so we don't break
+  it in a future version unintentionally,
+- Commit your code, but do not mess with `CHANGES.md`,
+
+Running tests
+-------------
+
+Make sure that you don't break anything with your changes by running:
+
+```bash
+$> composer install --dev --prefer-dist
+$> vendor/bin/phpspec run
+```