Gemfile: stable version with latest version of gems

closes #1234

Author: aollier

Gemfile

@@ -1,13 +1,13 @@
 source 'https://rubygems.org'
 
 gem 'rake'
-gem 'asciidoctor', '1.5.6.1'
+gem 'asciidoctor', '~> 2.0', '>= 2.0.10'
 
 gem 'json'
 gem 'awesome_print'
 
-gem 'asciidoctor-epub3', :git => 'https://github.com/asciidoctor/asciidoctor-epub3'
-gem 'asciidoctor-pdf', '1.5.0.alpha.16'
+gem 'asciidoctor-epub3', '~> 1.5.0.alpha.9'
+gem 'asciidoctor-pdf', '~> 1.5.0.beta.8'
 
 gem 'coderay'
 gem 'pygments.rb'