Bump PHP version constraint to match 2.4.0 (#28)

Author: tjwiebell

composer.json

@@ -5,7 +5,7 @@
         "sort-packages": true
     },
     "require": {
-        "php": "~7.1.3||~7.2.0||~7.3.0",
+        "php": "~7.3.0||~7.4.0",
         "magento/framework": "*",
         "magento/upward": "*"
     },