[BUGFIX] Add PHP version constraint to Composer manifest

This prevents installation in environments still using PHP 5.6.

composer.json

@@ -15,6 +15,7 @@
 		}
 	],
 	"require": {
+	    "php": ">=7.0.0 <7.2",
 		"typo3/cms-core": "7.6.0 - 8.7.99",
 		"sgalinski/tinymce": "^5.0"
 	},