Updates branch aliases in preparation for release.

- dev-master => 2.0.x-dev
- dev-develop => 2.1.x-dev
- Adds dev-release-1.8 => release-1.8 (for tracking LTS)

Author: weierophinney

composer.json

@@ -20,8 +20,9 @@
   },
   "extra": {
     "branch-alias": {
-      "dev-master": "1.8.x-dev",
-      "dev-develop": "2.0.x-dev"
+      "dev-master": "2.0.x-dev",
+      "dev-develop": "2.1.x-dev",
+      "dev-release-1.8": "1.8.x-dev"
     }
   },
   "require": {