Bump rubocop-govuk from 3.14.0 to 3.15.0

Bumps [rubocop-govuk](https://github.com/alphagov/rubocop-govuk) from 3.14.0 to 3.15.0.
- [Release notes](https://github.com/alphagov/rubocop-govuk/releases)
- [Changelog](https://github.com/alphagov/rubocop-govuk/blob/master/CHANGELOG.md)
- [Commits](alphagov/rubocop-govuk@v3.14.0...v3.15.0)

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

Gemfile.lock

@@ -180,7 +180,7 @@ GEM
       mini_portile2 (~> 2.4.0)
     null_logger (0.0.1)
     parallel (1.19.1)
-    parser (2.7.1.2)
+    parser (2.7.1.3)
       ast (~> 2.4.0)
     phantomjs (2.1.1.0)
     plek (3.0.0)
@@ -247,15 +247,19 @@ GEM
     rexml (3.2.4)
     robotex (1.0.0)
     rouge (3.19.0)
-    rubocop (0.83.0)
+    rubocop (0.85.0)
       parallel (~> 1.10)
       parser (>= 2.7.0.1)
       rainbow (>= 2.2.2, < 4.0)
+      regexp_parser (>= 1.7)
       rexml
+      rubocop-ast (>= 0.0.3)
       ruby-progressbar (~> 1.7)
       unicode-display_width (>= 1.4.0, < 2.0)
-    rubocop-govuk (3.14.0)
-      rubocop (= 0.83.0)
+    rubocop-ast (0.0.3)
+      parser (>= 2.7.0.1)
+    rubocop-govuk (3.15.0)
+      rubocop (= 0.85.0)
       rubocop-rails (~> 2)
       rubocop-rake (~> 0.5.1)
       rubocop-rspec (~> 1.28)