Bump html-proofer from 3.14.1 to 3.15.0

Bumps html-proofer from 3.14.1 to 3.15.0.

Release notes

Sourced from html-proofer's releases.

v3.15.0

• gjtorikian/html-proofer#362

Commits

• 9de2624 💎 bump to 3.15.0
• 1471239 Merge pull request #362 from jeremy/html5
• 95edfa8 rubocop!!
• 3ef5af5 Add missing docs
• a71aac0 Slight gem fixes
• 14dd2f2 Add doctype to valid doc fixture
• 891df75 rubocop
• ac93f46 Merge branch 'master' into html5
• 4f94fed CI trigger?
• 72a4421 Report errors
• Additional commits viewable in compare view