Bump html-proofer from 3.18.6 to 3.18.8

Bumps html-proofer from 3.18.6 to 3.18.8.

Commits

• 8d606b0 💎 bump to 3.18.8
• e1c7d08 require
• 5d03743 💎 bump to 3.18.7
• b983c35 Merge pull request #628 from gjtorikian/request-fix
• 902fbd1 Explicitly state type when retrieving cached URLs
• 45c85f4 Demonstrate runner/cache conflict
• b655dbf Merge pull request #626 from gjtorikian/link-fun
• c9128c4 Do a better job preserving cache logic
• See full diff in compare view