fix asciidoctor to the latest stable

jnavila · jnavila · commit e73d8b116104 · 2018-01-31T20:58:57.000+01:00
This helps reproducible build and fixes a regression in the latest master.
diff --git a/Gemfile b/Gemfile
@@ -1,7 +1,7 @@
 source 'https://rubygems.org'
 
 gem 'rake'
-gem 'asciidoctor', :git => 'https://github.com/asciidoctor/asciidoctor'
+gem 'asciidoctor', '1.5.6.1'
 
 gem 'json'
 gem 'awesome_print'
