prevent installing with symfony/http-foundation 4 as we are not compatible with that

dbu · dbu · commit ec3de5f1ac07 · 2018-02-07T18:03:10.000+01:00
diff --git a/composer.json b/composer.json
@@ -41,6 +41,9 @@
             "FOS\\HttpCache\\Tests\\": "tests/"
         }
     },
+    "conflict": {
+        "symfony/http-foundation": ">=4.0"
+    },
     "extra": {
         "branch-alias": {
             "dev-master": "1.4.x-dev"
