converters/cbmconvert: update to 2.1.5

This is a maintenance release of cbmconvert, with no new features.
What's Changed

  - REL files are correctly written to Lynx archives.
  - 1571 images can be actually created.
  - Most code is now covered by CTest.
  - ARC/SDA, Arkive, T64 are not covered at all yet.

Full Changelog: https://github.com/dr-m/cbmconvert/commits/cbmconvert-2.1.5

(rhialto)