Upgrade Guava to 20.0-rc1

The propagate and propagateIfPossible methods are deprecated. Replace
them with the recommended alternatives.

Change-Id: I9107dc51b968e2acc453502ece3c5ed5dd58b5de

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

@@ -399,7 +399,7 @@ 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());
       }
     }