Allow to use with nokogiri ~> 1.6.0
Allow to use with nokogiri ~> 1.6.0

But users should still avoid >=1.6.4 and <1.6.6 which are broken.
Today's version 1.6.6.1 is okay.

rahmanusta/asciidoctor-doctest,asciidoctor/asciidoctor-doctest,rahmanusta/asciidoctor-doctest,asciidoctor/asciidoctor-doctest