lock php version

composer.json

@@ -1,7 +1,9 @@
 {
     "name": "medieval-inc/quadratic",
     "type": "library",
-    "require": {},
+    "require": {
+        "php" : "^7.2"
+    },
     "require-dev": {
         "phpunit/phpunit": "^7.3"
     },

composer.lock

@@ -4,7 +4,7 @@
         "Read more about it at https://getcomposer.org/doc/01-basic-usage.md#installing-dependencies",
         "This file is @generated automatically"
     ],
-    "content-hash": "7ad4921e67c1bb9fc7e6e10147d28360",
+    "content-hash": "54bcf18af402773825d38eed23ebd229",
     "packages": [],
     "packages-dev": [
         {
@@ -1418,6 +1418,8 @@
     "stability-flags": [],
     "prefer-stable": false,
     "prefer-lowest": false,
-    "platform": [],
+    "platform": {
+        "php": "^7.2"
+    },
     "platform-dev": []
 }