Add Composer allow-plugins config (#89)

* Add Composer allow-plugins config * Fix indentation and trailing comma * Composer: consolidate config keys

Author: GaryJones

composer.json

@@ -21,7 +21,10 @@
  "source": "https://github.com/sirbrillig/phpcs-neutron-standard"
  },
  "config": {
- "sort-order": true
+ "sort-order": true,
+ "allow-plugins": {
+ "dealerdirect/phpcodesniffer-composer-installer": true
+ }
  },
  "autoload": {
  "psr-4": {