Bump html-proofer from 3.14.1 to 3.15.0

Bumps [html-proofer](https://github.com/gjtorikian/html-proofer) from 3.14.1 to 3.15.0.
- [Release notes](https://github.com/gjtorikian/html-proofer/releases)
- [Commits](gjtorikian/html-proofer@v3.14.1...v3.15.0)

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

Gemfile.lock

@@ -91,10 +91,10 @@ GEM
     html-pipeline (2.12.2)
       activesupport (>= 2)
       nokogiri (>= 1.4)
-    html-proofer (3.14.1)
+    html-proofer (3.15.0)
       addressable (~> 2.3)
       mercenary (~> 0.3)
-      nokogiri (~> 1.10)
+      nokogumbo (~> 2.0)
       parallel (~> 1.3)
       rainbow (~> 3.0)
       typhoeus (~> 1.3)
@@ -223,9 +223,11 @@ GEM
     multipart-post (2.1.1)
     nokogiri (1.10.7)
       mini_portile2 (~> 2.4.0)
+    nokogumbo (2.0.2)
+      nokogiri (~> 1.8, >= 1.8.4)
     octokit (4.14.0)
       sawyer (~> 0.8.0, >= 0.5.3)
-    parallel (1.19.0)
+    parallel (1.19.1)
     parser (2.6.5.0)
       ast (~> 2.4.0)
     pathutil (0.16.2)