Bump mocha from 1.14.0 to 1.15.0

Bumps [mocha](https://github.com/freerange/mocha) from 1.14.0 to 1.15.0.
- [Release notes](https://github.com/freerange/mocha/releases)
- [Changelog](https://github.com/freerange/mocha/blob/main/RELEASE.md)
- [Commits](freerange/mocha@v1.14.0...v1.15.0)

---
updated-dependencies:
- dependency-name: mocha
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

Gemfile.lock

@@ -186,7 +186,7 @@ GEM
       capybara
       minitest (~> 5.0)
       rake
-    mocha (1.14.0)
+    mocha (1.15.0)
     net-imap (0.2.3)
       digest
       net-protocol