Fix "jekyll build" RuntimeError#18

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tonyduckles wants to merge 2 commits intooctopress:masterfrom

tonyduckles:jekyll-compat

Commits on Feb 13, 2016

Changed access to the extension of an item, so Jekyll doesn't emit deprecation-warnings anymore
maxkandler
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Commits on Jan 26, 2018

Fix Jekyll compatibility: lib/octopress-codefence.rb:26:in `encode!': can't modify frozen String (RuntimeError)
tonyduckles
committed