Bump slimmer from 13.0.0 to 13.1.0

Bumps [slimmer](https://github.com/alphagov/slimmer) from 13.0.0 to 13.1.0.
- [Release notes](https://github.com/alphagov/slimmer/releases)
- [Changelog](https://github.com/alphagov/slimmer/blob/master/CHANGELOG.md)
- [Commits](alphagov/slimmer@v13.0.0...v13.1.0)

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

Author: dependabot-support

Gemfile

@@ -19,7 +19,7 @@ gem 'govuk_app_config', '~> 1.11'
 gem 'govuk_frontend_toolkit', '~> 8.1.0'
 gem 'govuk_publishing_components', '~> 13.5.2'
 gem 'plek', '~> 2.1'
-gem 'slimmer', '~> 13.0'
+gem 'slimmer', '~> 13.1'
 
 group :development, :test do
   gem 'govuk-lint'

Gemfile.lock

@@ -191,7 +191,7 @@ GEM
     multipart-post (2.0.0)
     netrc (0.11.0)
     nio4r (2.3.1)
-    nokogiri (1.9.1)
+    nokogiri (1.10.1)
       mini_portile2 (~> 2.4.0)
     nokogumbo (1.5.0)
       nokogiri
@@ -303,7 +303,7 @@ GEM
       rubyzip (~> 1.2, >= 1.2.2)
     sentry-raven (2.7.4)
       faraday (>= 0.7.6, < 1.0)
-    slimmer (13.0.0)
+    slimmer (13.1.0)
       activesupport
       json
       nokogiri (~> 1.7)
@@ -385,7 +385,7 @@ DEPENDENCIES
   rails_translation_manager (~> 0.1.0)
   sass-rails (~> 5.0)
   selenium-webdriver
-  slimmer (~> 13.0)
+  slimmer (~> 13.1)
   uglifier (>= 1.3.0)
   webmock (~> 3.5.1)
   wraith (~> 4.2)