bug #4552 required PHPUnit version in the docs should be updated to 4.2 (or later)... (jzawadzki)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #4552).

Discussion
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required PHPUnit version in the docs should be updated to 4.2 (or later)...

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.6
| Fixed tickets | #4547

Commits
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bf8cee6 required PHPUnit version in the docs should be updated to 4.2 (or later) since the atLeast() method which is used in some of the tests

Author: weaverryan

contributing/code/tests.rst

@@ -9,7 +9,7 @@ Symfony test suite to check that you have not broken anything.
 PHPUnit
 -------
 
-To run the Symfony test suite, `install PHPUnit`_ 3.7 (or later) first.
+To run the Symfony test suite, `install PHPUnit`_ 4.2 (or later) first.
 
 Dependencies (optional)
 -----------------------