make it compile in SBT & Eclipse with 2.9.2-SNAPSHOT

- there is one remaining bug, which makes it impossible to implement
  Function1[T, Boolean] from Java (I wonder if this will ever work,
  given that there is no equivalent type expressible in Java)
- hence create Function1 in Scala code, Java only prepares a Filter SAM
  which does not inherit from anything
- ugly (one more alloc) but the only way I see right now.

akka-docs/java/code/akka/docs/future/FutureDocTestBase.java

@@ -317,17 +317,17 @@ public class FutureDocTestBase {
   public void useFilter() throws Exception {
     //#filter
     Future<Integer> future1 = Futures.successful(4, system.dispatcher());
-    Future<Integer> successfulFilter = future1.filter(new Filter<Integer>() {
+    Future<Integer> successfulFilter = future1.filter(Filter.create(new Filter<Integer>() {
       public boolean filter(Integer i) {
         return i % 2 == 0;
       }
-    });
+    }));
 
-    Future<Integer> failedFilter = future1.filter(new Filter<Integer>() {
+    Future<Integer> failedFilter = future1.filter(Filter.create(new Filter<Integer>() {
       public boolean filter(Integer i) {
         return i % 2 != 0;
       }
-    });
+    }));
     //When filter fails, the returned Future will be failed with a scala.MatchError
     //#filter
   }