Fix versions upgrade with PostgreSQL and implicit casts

Version upgrades sometimes fails:

org.nuxeo.ecm.core.storage.StorageException: org.postgresql.util.PSQLException: ERROR: operator is not unique: integer || unknown
Hint: Could not choose a best candidate operator. You might need to add explicit type casts.
Context: PL/pgSQL function "nx_upgrade_versions" line 21 at SQL statement

ERROR: operator is not unique: integer || unknown at character 34
HINT: Could not choose a best candidate operator. You might need to add explicit type casts.
QUERY: UPDATE versions SET label = $1 || '.' || $2 , islatest = $3 , islatestmajor = $4 AND $5 WHERE id = $6
CONTEXT: PL/pgSQL function "nx_upgrade_versions" line 21 at SQL statement
STATEMENT: SELECT nx_upgrade_versions()