minor #841 Fix installation when using composer 1.7+ (fbourigault)

This PR was merged into the master branch.

Discussion
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Fix installation when using composer 1.7+

This avoid the following error while installing the demo application with composer 1.7.

```
Declaration of Symfony\Flex\ParallelDownloader::getRemoteContents($originUrl, $fileUrl, $context) should be compatible with Composer\Util\RemoteFilesystem::getRemoteContents
  ($originUrl, $fileUrl, $context, ?array &$responseHeaders = NULL)
```

Commits
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14ba756 fix installation when using composer 1.7+

composer.json

@@ -15,7 +15,7 @@
         "sensiolabs/security-checker": "^4.1",
         "symfony/asset": "^4.1",
         "symfony/expression-language": "^4.1",
-        "symfony/flex": "^1.0",
+        "symfony/flex": "^1.0.86",
         "symfony/form": "^4.1",
         "symfony/framework-bundle": "^4.1",
         "symfony/monolog-bundle": "^3.1",