Bump slimmer from 15.4.0 to 15.4.1

Bumps [slimmer](https://github.com/alphagov/slimmer) from 15.4.0 to 15.4.1.
- [Release notes](https://github.com/alphagov/slimmer/releases)
- [Changelog](https://github.com/alphagov/slimmer/blob/master/CHANGELOG.md)
- [Commits](alphagov/slimmer@v15.4.0...v15.4.1)

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

Gemfile.lock

@@ -242,7 +242,7 @@ GEM
     middleware (0.1.0)
     mime-types (3.3.1)
       mime-types-data (~> 3.2015)
-    mime-types-data (3.2020.1104)
+    mime-types-data (3.2021.0212)
     mimemagic (0.3.5)
     mini_mime (1.0.2)
     mini_portile2 (2.5.0)
@@ -418,7 +418,7 @@ GEM
       docile (~> 1.1)
       simplecov-html (~> 0.11)
     simplecov-html (0.12.2)
-    slimmer (15.4.0)
+    slimmer (15.4.1)
       activesupport
       json
       nokogiri (~> 1.7)