Update composer/installers dependency requirement (#202)

This PR updates the composer/installers dependency requirement to allow installation of the 2.x branch.

Author: SteelWagstaff

composer.json

@@ -17,7 +17,7 @@
   },
   "require": {
     "php": ">=5.2",
-    "composer/installers": "^1.0"
+    "composer/installers": "^1.0 || ^2.0"
   },
   "require-dev": {
     "phpunit/phpunit": ">4.8.20 <6.0",