Use TreeMaps instead of Maps in a couple of places.

OpenJDK doesn't seem to like the latter, and hasn't been able to
compile for the last couple of days. SunJDK doesn't seem to have
any problems with swapping out Map for TreeMap, so we'll be more
explicit.

Change-Id: I4c0b9eeceed57974785c1598e2d6aff4b04ead10

gerrit-server/src/main/java/com/google/gerrit/server/change/ReviewerJson.java

@@ -38,7 +38,7 @@ import com.google.inject.Provider;
 
 import java.util.Collection;
 import java.util.List;
-import java.util.Map;
+import java.util.TreeMap;
 
 public class ReviewerJson {
   private final Provider<ReviewDb> db;
@@ -132,7 +132,7 @@ public class ReviewerJson {
 
   public static class ReviewerInfo extends AccountInfo {
     final String kind = "gerritcodereview#reviewer";
-    Map<String, String> approvals;
+    TreeMap<String, String> approvals;
 
     protected ReviewerInfo(Account.Id id) {
       super(id);