Try jekyll 2 again

This reverts #990, placing @nicolasgarnier's #977 back in place, and fixes the problem with Jekyll 2: whitespace on the first line files' metadata :frowning: \*wah wah\*

This is staged here: https://try-jekyll-2-again-dot-dart-lang.appspot.com

The following pages are now available:

* https://try-jekyll-2-again-dot-dart-lang.appspot.com/articles/writing-unit-tests-for-pub-packages/
* https://try-jekyll-2-again-dot-dart-lang.appspot.com/articles/benchmarking/
* https://try-jekyll-2-again-dot-dart-lang.appspot.com/articles/simd/
* https://try-jekyll-2-again-dot-dart-lang.appspot.com/logos