move async to 2.11.x branch

Author: SethTisue

common.conf

@@ -69,7 +69,7 @@ vars: {
 
   // tracking upstream (the ideal)
   akka-ref                     : "akka/akka.git#release-2.3"
-  scala-async-ref              : "scala/scala-async.git"
+  scala-async-ref              : "scala/scala-async.git#2.11.x"  // master dropped 2.11 support
   scala-parser-combinators-ref : "scala/scala-parser-combinators.git#1.1.x"
   scala-partest-ref            : "scala/scala-partest.git#1.0.x"
   scala-partest-interface-ref  : "scala/scala-partest-interface.git"