Bump html-proofer from 3.14.1 to 3.15.0 (#1604)

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>

Author: dependabot-preview[bot]

Gemfile.lock

@@ -11,12 +11,12 @@ GEM
     ethon (0.12.0)
       ffi (>= 1.3.0)
     eventmachine (1.2.7)
-    ffi (1.11.2)
+    ffi (1.11.3)
     forwardable-extended (2.6.0)
-    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)
@@ -53,9 +53,11 @@ GEM
       ruby_dep (~> 1.2)
     mercenary (0.3.6)
     mini_portile2 (2.4.0)
-    nokogiri (1.10.5)
+    nokogiri (1.10.7)
       mini_portile2 (~> 2.4.0)
-    parallel (1.19.0)
+    nokogumbo (2.0.2)
+      nokogiri (~> 1.8, >= 1.8.4)
+    parallel (1.19.1)
     pathutil (0.16.2)
       forwardable-extended (~> 2.6)
     public_suffix (4.0.1)