minor #15173 [2.8] Fix lowest deps (nicolas-grekas)

This PR was merged into the 2.8 branch.

Discussion
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[2.8] Fix lowest deps

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

Looks like that by testing the lowest deps on 5.3, we missed some constraints in our test suite.

Commits
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a036a77 [2.8] Fix lowest deps

Author: nicolas-grekas

composer.json

@@ -23,6 +23,7 @@
     "require-dev": {
         "symfony/phpunit-bridge": "~2.7|~3.0.0",
         "symfony/browser-kit": "~2.4|~3.0.0",
+        "symfony/config": "~2.8|~3.0.0",
         "symfony/console": "~2.7|~3.0.0",
         "symfony/css-selector": "~2.0,>=2.0.5|~3.0.0",
         "symfony/dependency-injection": "~2.6,>=2.6.6|~3.0.0",