avoid sql error when updating sql database

The index doesn't exist in version 1.7.6 and so trying to drop it
would lead to an error, resulting in not being able to import
1.7.6 database.
https://github.com/iSoron/uhabits/issues/327

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;