minor symfony#20894 [HttpKernel] Fix missing psr/cache dev requirement (chalasr)

This PR was merged into the 3.3-dev branch.

Discussion
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[HttpKernel] Fix missing psr/cache dev requirement

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | yes, tests are broken
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

Because symfony#20810 added a test for the `Psr6CacheClearer` that mocks methods of the `CacheItemPoolInterface`. It should fix the travis build on master and deps=low

Commits
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bd59d75 [HttpKernel] Require psr/cache in dev

Author: nicolas-grekas

src/Symfony/Component/HttpKernel/composer.json

@@ -37,7 +37,8 @@
         "symfony/stopwatch": "~2.8|~3.0",
         "symfony/templating": "~2.8|~3.0",
         "symfony/translation": "~2.8|~3.0",
-        "symfony/var-dumper": "~3.2"
+        "symfony/var-dumper": "~3.2",
+        "psr/cache": "~1.0"
     },
     "conflict": {
         "symfony/config": "<2.8"