Merge pull request #333 from llunak/fix_sql

avoid sql error when updating sql database

uhabits-core/src/main/resources/migrations/22.sql

@@ -13,7 +13,7 @@ begin transaction;
         habit integer not null references habits(id),
         timestamp integer not null,
         value integer not null);
-    drop index idx_repetitions_habit_timestamp;
+    drop index if exists idx_repetitions_habit_timestamp;
     create unique index idx_repetitions_habit_timestamp on Repetitions(
         habit, timestamp);
     insert into Repetitions select * from RepetitionsBak;