minor #20843 [Config] fix dev dependencies (xabbuh)

This PR was merged into the 2.7 branch.

Discussion
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[Config] fix dev dependencies

| Q             | A
| ------------- | ---
| Branch?       | 2.7
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #20827
| License       | MIT
| Doc PR        |

Since #20827, we need the Yaml component to run the tests for the `YamlReferenceDumper`.

Commits
-------

292ae7c [Config] fix dev dependencies

src/Symfony/Component/Config/composer.json

@@ -19,6 +19,9 @@
         "php": ">=5.3.9",
         "symfony/filesystem": "~2.3"
     },
+    "require-dev": {
+        "symfony/yaml": "~2.7"
+    },
     "suggest": {
         "symfony/yaml": "To use the yaml reference dumper"
     },