Bump bundled libverto for 0.3.0 release

Bump bundled libverto for 0.3.0 release

Local changes:
  - Update upstream URL for fedorahosted deprecation.
  - Add verto_cleanup() to exports.

Upstream changes:
  - Strict c89 compliance.
  - Remove local asprintf() implementation.
  - Fix memleak when memory allocator is realloc().
  - Fix leaks of filenames during load.
  - Fix many unused variable warnings.
  - Factor out mutability check.
  - Properly handle _GNU_SOURCE checking.

ticket: 8612 (new)