Bump html-proofer from 3.18.5 to 3.18.6

Bumps html-proofer from 3.18.5 to 3.18.6.

Commits

• 75ad787 💎 bump to 3.18.6
• 925a2b0 Merge pull request #623 from gjtorikian/internal-drama
• ed39330 lint
• 580c6c1 each_with_object forever
• 8a186cb typo
• aa60148 Handle internal & external cache merging
• 98554c8 Merge pull request #622 from gjtorikian/fix-middleware-url-ignore
• 777d834 Correct link ignore regexp
• See full diff in compare view