BUGFIX: With Neos 4.x psr7 is required as dependency

Author: Sebobo

composer.json

@@ -4,7 +4,8 @@
     "name": "flowpack/fullpagecache",
     "license": "MIT",
     "require": {
-        "neos/neos": "~4.3.6 || ^5.0 || dev-master"
+        "neos/neos": "~4.3.6 || ^5.0 || dev-master",
+        "guzzlehttp/psr7": "^1.4"
     },
     "autoload": {
         "psr-4": {