try here

Gemfile

@@ -10,5 +10,6 @@ gem "rdoc", "~>3.6"
 gem "org-ruby", "= 0.9.9"
 gem "creole", "~>0.3.6"
 gem "wikicloth", "=0.8.3"
+gem "twitter-text", "~> 1.14"
 gem "asciidoctor", "= 1.5.6.1"
 gem "rake"

github-markup.gemspec

@@ -24,5 +24,4 @@ Gem::Specification.new do |s|
   s.add_development_dependency 'nokogiri', '1.6.8.1'
   s.add_development_dependency 'nokogiri-diff', '~> 0.2.0'
   s.add_development_dependency "github-linguist", "~> 6.0"
-  s.add_development_dependency "twitter-text", "~> 1.14"
 end