Bump slimmer from 15.5.0 to 15.5.1

Bumps [slimmer](https://github.com/alphagov/slimmer) from 15.5.0 to 15.5.1.
- [Release notes](https://github.com/alphagov/slimmer/releases)
- [Changelog](https://github.com/alphagov/slimmer/blob/master/CHANGELOG.md)
- [Commits](alphagov/slimmer@v15.5.0...v15.5.1)

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

Author: dependabot[bot]

Gemfile.lock

@@ -307,7 +307,7 @@ GEM
       docile (~> 1.1)
       simplecov-html (~> 0.11)
     simplecov-html (0.12.3)
-    slimmer (15.5.0)
+    slimmer (15.5.1)
       activesupport
       json
       nokogiri (~> 1.7)