Sets branch alias for release-1.8 branch

- Lists only release-1.8 (mapping to 1.8.x-dev), as we do not want to
  update it on each and every 2.Y release.

Author: weierophinney

composer.json

@@ -18,9 +18,7 @@
   },
   "extra": {
     "branch-alias": {
-      "dev-master": "1.8.x-dev",
-      "dev-develop": "1.9.x-dev",
-      "dev-release-2.0": "2.0.x-dev"
+      "dev-release-1.8": "1.8.x-dev"
     }
   },
   "require": {