switch composer to php7 only

Author: gregor-j

composer.json

@@ -17,7 +17,7 @@
   ],
   "minimum-stability": "stable",
   "require": {
-    "php": "^5.5|^7.0",
+    "php": "^7.0",
     "ext-json": "*",
     "php-sap/interfaces": "^2.0",
     "php-sap/datetime": "^1.2"