build(deps): bump mustermann from 3.0.0 to 3.0.4

Bumps [mustermann](https://github.com/sinatra/mustermann) from 3.0.0 to 3.0.4.
- [Commits](sinatra/mustermann@v3.0.0...v3.0.4)

---
updated-dependencies:
- dependency-name: mustermann
  dependency-version: 3.0.4
  dependency-type: indirect
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]

Gemfile.lock

@@ -317,7 +317,7 @@ GEM
     multipart-parser (0.1.1)
     multipart-post (2.4.1)
     mustache (1.1.1)
-    mustermann (3.0.0)
+    mustermann (3.0.4)
       ruby2_keywords (~> 0.0.1)
     mutex_m (0.3.0)
     mysql2 (0.5.7)