GeezerVison

Follow-up to #251. I checked every external link in the README; omniref was the only fully dead one, but a few others were stale or only worked through redirects.

Changes

• Unraveling String Key Performance in Ruby 2.2: the SitePoint URL now redirects to "Crafting Ruby for Performance", an unrelated article. Points to the archived original on the Wayback Machine instead.
• Derailed Benchmarks: schneems/derailed_benchmarks -> zombocom/derailed_benchmarks (the repo moved).
• Weird stuff with hashes: tenderlovemaking.com moved the post from .html to a trailing-slash path.
• Switch the remaining http:// links (ruby-doc.org, Rails 5.1 guides, SitePoint Symbol GC) to https://.

Verification

Each changed link returns 200 with no redirects, and the archived SitePoint page is the original article.

Not changed: the "Share this" tweet link still uses twitter.com and git.io. Both still redirect to the right place.