Generic covariance of concat.

See ReactiveX#360 (comment)

rxjava-core/src/main/java/rx/Observable.java

@@ -1199,7 +1199,7 @@ public static <T> Observable<T> merge(Observable<? extends T> t1, Observable<? e
      *         the {@code source} Observables, one after the other
      * @see <a href="http://msdn.microsoft.com/en-us/library/system.reactive.linq.observable.concat(v=vs.103).aspx">MSDN: Observable.Concat Method</a>
      */
-    public static <T> Observable<T> concat(Observable<Observable<T>> observables) {
+    public static <T> Observable<T> concat(Observable<? extends Observable<? extends T>> observables) {
         return create(OperationConcat.concat(observables));
     }