Open the backup file and extract the dump.sql file from the dump.sql.gz. Open the dump.sql file and look for a block of numbers that gets inserted into the schema_migrations table.
COPY public.schema_migrations (version) FROM stdin;
20000225050318
20120311163914
20120311164326
...
20181031165343
20181112013117
20181120140552
\.
Take the highest number and append it to your backup file. Make sure to prefix the number with -v. For example, it might look like this: mybackup-v20181120140552.tar.gz