minor symfony#46287 Allow flex plugin + remove travis-ci file (nicolas-grekas)

This PR was merged into the 4.4 branch.

Discussion
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Allow flex plugin + remove travis-ci file

| Q             | A
| ------------- | ---
| Branch?       | 4.4
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Tickets       | -
| License       | MIT
| Doc PR        | -

Commits
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1c90d84 Allow flex plugin + remove travis-ci file

Author: nicolas-grekas

.github/composer-config.json

@@ -7,6 +7,9 @@
             "symfony/proxy-manager-bridge": "source",
             "symfony/validator": "source",
             "*": "dist"
+        },
+        "allow-plugins": {
+            "symfony/flex": true
         }
     }
 }