Bump mnemosyne-ruby from 1.14.0 to 1.15.0

Bumps [mnemosyne-ruby](https://github.com/jgraichen/mnemosyne-ruby) from 1.14.0 to 1.15.0.
- [Release notes](https://github.com/jgraichen/mnemosyne-ruby/releases)
- [Changelog](https://github.com/mnemosyne-mon/mnemosyne-ruby/blob/main/CHANGELOG.md)
- [Commits](https://github.com/jgraichen/mnemosyne-ruby/compare/v1.14.0...v1.15.0)

---
updated-dependencies:
- dependency-name: mnemosyne-ruby
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Gemfile.lock

@@ -225,7 +225,7 @@ GEM
     mini_mime (1.1.5)
     mini_portile2 (2.8.4)
     minitest (5.19.0)
-    mnemosyne-ruby (1.14.0)
+    mnemosyne-ruby (1.15.0)
       activesupport (>= 4)
       bunny
     msgpack (1.7.2)