Remove usage of to-be-deprecated Throwables

The propagate and propagateIfPossible methods will be deprecated
in Guava 20. Replace them with the recommended alternatives.

Change-Id: I1c8d19aac5434c938ee10af88766af14e5e4d023

gerrit-lucene/src/main/java/com/google/gerrit/lucene/LuceneChangeIndex.java

@@ -377,7 +377,7 @@ public class LuceneChangeIndex implements ChangeIndex {
         close();
         throw new OrmRuntimeException(e);
       } catch (ExecutionException e) {
-        Throwables.propagateIfPossible(e.getCause());
+        Throwables.throwIfUnchecked(e.getCause());
         throw new OrmRuntimeException(e.getCause());
       }
     }