Change php requirement and symfony/cache version

Signed-off-by: Lasse Mammen <lkm@bookboon.com>

Author: lkm

composer.json

@@ -14,11 +14,11 @@
         }
     ],
     "require": {
-        "php": ">=5.6",
+        "php": ">=7.1",
         "symfony/dependency-injection": "~4.0",
         "symfony/config": "~4.0",
-        "bookboon/api": "~4.0",
-        "symfony/cache": "^4.2"
+        "symfony/cache": "~4.0",
+        "bookboon/api": "~4.0"
     },
     "autoload": {
         "psr-4": {