Update composer.json

Allow updates for `composer/installers`.

Author: villesiltala

composer.json

@@ -20,7 +20,7 @@
   "require": {
     "php": ">=5.4",
     "devgeniem/dustpress": ">=0.4.0.2",
-    "composer/installers": "v1.0.12"
+    "composer/installers": "^1.0.12"
   },
   "keywords": [
     "wordpress", "plugin", "dustjs", "dustpress", "debugger", "theme", "framework", "javascript"