Fix composer require version

tenkoma · sebastianbergmann · commit fc46957e62d0 · 2018-01-29T15:55:33.000+01:00
diff --git a/src/installation.rst b/src/installation.rst
@@ -274,7 +274,7 @@ dependencies of your project:
 
 .. code-block:: bash
 
-    composer require --dev phpunit/phpunit ^6.5
+    composer require --dev phpunit/phpunit ^7.0
 
 .. _installation.optional-packages:
 
