Bump html-proofer from 3.15.0 to 3.15.1

Bumps [html-proofer](https://github.com/gjtorikian/html-proofer) from 3.15.0 to 3.15.1.
- [Release notes](https://github.com/gjtorikian/html-proofer/releases)
- [Commits](gjtorikian/html-proofer@v3.15.0...v3.15.1)

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

Author: dependabot-preview[bot]

Gemfile.lock

@@ -28,7 +28,7 @@ GEM
     execjs (2.7.0)
     faraday (0.17.1)
       multipart-post (>= 1.2, < 3)
-    ffi (1.11.3)
+    ffi (1.12.1)
     forwardable-extended (2.6.0)
     gemoji (3.0.1)
     github-pages (203)
@@ -83,7 +83,7 @@ GEM
     html-pipeline (2.12.3)
       activesupport (>= 2)
       nokogiri (>= 1.4)
-    html-proofer (3.15.0)
+    html-proofer (3.15.1)
       addressable (~> 2.3)
       mercenary (~> 0.3)
       nokogumbo (~> 2.0)