Bump rails_admin from 3.1.1 to 3.1.2

Bumps [rails_admin](https://github.com/sferik/rails_admin) from 3.1.1 to 3.1.2.
- [Release notes](https://github.com/sferik/rails_admin/releases)
- [Changelog](https://github.com/railsadminteam/rails_admin/blob/master/CHANGELOG.md)
- [Commits](https://github.com/sferik/rails_admin/compare/v3.1.1...v3.1.2)

---
updated-dependencies:
- dependency-name: rails_admin
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

Gemfile.lock

@@ -326,7 +326,7 @@ GEM
     rails-timeago (2.20.0)
       actionpack (>= 5.2)
       activesupport (>= 5.2)
-    rails_admin (3.1.1)
+    rails_admin (3.1.2)
       activemodel-serializers-xml (>= 1.0)
       kaminari (>= 0.14, < 2.0)
       nested_form (~> 0.3)