minor #900 Add the Symfony CSSSelector dependency to match the example usage (tzi)

This PR was merged into the 1.2.x-dev branch.

Discussion
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Add the Symfony CSSSelector dependency to match the example usage

Hi!

I added the Symfony CSSSelector dependency to match with the example usage & the crawler presentation.

Cheers,
Thomas.

Commits
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4c7e740e Add the Symfony CSSSelector dependency to match the example usage

doc/testing.rst

@@ -85,7 +85,8 @@ use it by making your test extend it::
     .. code-block:: json
 
         "require-dev": {
-            "symfony/browser-kit": ">=2.3,<2.4-dev"
+            "symfony/browser-kit": ">=2.3,<2.4-dev",
+            "symfony/css-selector": ">=2.3,<2.4-dev"
         }
 
 For your WebTestCase, you will have to implement a ``createApplication``