Build/Test Tools: Specify PHP 5.6 as the required version of PHP in Composer.

Props ayeshrajans, jrf, netweb.
Fixes #48081.

git-svn-id: https://develop.svn.wordpress.org/trunk@46200 602fd350-edb4-49c9-b593-d223f7449a82

composer.json

@@ -9,6 +9,9 @@
 	"support": {
 		"issues": "https://core.trac.wordpress.org/"
 	},
+	"require": {
+		"php": ">=5.6"
+	},
 	"require-dev": {
 		"dealerdirect/phpcodesniffer-composer-installer": "~0.5.0",
 		"wp-coding-standards/wpcs": "~2.1.0"

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": "54fbee5534d4ee1d332bb374c5442e79",
+    "content-hash": "56c797c5309702adf0f49bd274a364f3",
     "packages": [],
     "packages-dev": [
         {
@@ -175,6 +175,8 @@
     "stability-flags": [],
     "prefer-stable": false,
     "prefer-lowest": false,
-    "platform": [],
+    "platform": {
+      "php": ">=5.6"
+    },
     "platform-dev": []
 }