minor #4669 Harmonize PHPUnit version to 4.2 or above (xelaris)

This PR was merged into the 2.3 branch.

Discussion
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Harmonize PHPUnit version to 4.2 or above

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | #4547

Use the same version as the one on the dedicated "tests" page since #4552 and link to phpunit.de

Commits
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df25c36 Harmonize PHPUnit version to 4.2 or above

contributing/code/patches.rst

@@ -15,7 +15,7 @@ software:
 
 * Git;
 * PHP version 5.3.3 or above;
-* PHPUnit 3.6.4 or above.
+* `PHPUnit`_ 4.2 or above.
 
 Configure Git
 ~~~~~~~~~~~~~
@@ -415,3 +415,4 @@ messages of all the commits. When you are finished, execute the push command.
 .. _`fabbot`:                              http://fabbot.io
 .. _`PSR-1`:                               http://www.php-fig.org/psr/psr-1/
 .. _`PSR-2`:                               http://www.php-fig.org/psr/psr-2/
+.. _PHPUnit:                               https://phpunit.de/manual/current/en/installation.html