Merge branch '3.4' into 4.3

* 3.4:
  Bump minimal requirements
  Use PHPUnit 8.3 on Travis when possible

composer.json

@@ -25,7 +25,7 @@
         "symfony/asset": "~3.4|~4.0",
         "symfony/dependency-injection": "~3.4|~4.0",
         "symfony/finder": "~3.4|~4.0",
-        "symfony/form": "^4.3",
+        "symfony/form": "^4.3.4",
         "symfony/http-foundation": "~4.3",
         "symfony/http-kernel": "~3.4|~4.0",
         "symfony/mime": "~4.3",