Add a branch alias

Add a branch alias to be able to require 3.0.* version (use the branch master if minimum-stability is dev or use a tag if it's defined to stable)

composer.json

@@ -17,4 +17,9 @@
       "issues": "https://github.com/twbs/bootstrap/issues"
     }
   , "license": "Apache-2.0"
+  , "extra": {
+      "branch-alias": {
+          "dev-master": "3.0.x-dev"
+      }
+  }
 }