remove future from trip reader after closing

Future was not removed from the reader instance after close() has been
called, so re-opening was not possible. Remove the instance after
termination allows calling open() again.

src/main/java/de/stklcode/pubtrans/ura/reader/AsyncUraTripReader.java

@@ -124,6 +124,8 @@ public class AsyncUraTripReader implements AutoCloseable {
         } catch (TimeoutException e) {
             // Task failed to finish within 1 second.
             future.cancel(true);
+        } finally {
+            future = null;
         }
     }