restore missing slim/psr7 dependency (#77)

Author: dzuelke

composer.json

@@ -4,6 +4,7 @@
     "php-di/php-di": "^7.0",
     "slim/slim": "^4.0",
     "php-di/slim-bridge": "^3.4.1",
+    "slim/psr7": "^1.3.0",
     "monolog/monolog": "^3.0",
     "twig/twig": "^3.0",
     "slim/twig-view": "^3.0"